feat(settings): theme "System" + reduce-motion (plan Tier 1)

- ThemeContext gains a 'system' theme that follows prefers-color-scheme and
  updates live when the OS flips; exposes resolvedTheme (consumed by Sonner) and
  keeps the <meta theme-color> in sync. Pre-bundle inline script in index.html
  prevents a light/dark flash. Both theme selectors (Settings ThemeCards +
  theme-toggle dropdown) gain the System option.
- New MotionPreferenceContext mounts framer-motion <MotionConfig>; a Reduce-motion
  toggle (localStorage device pref) forces reducedMotion="always" (else "user",
  which still respects the OS).
- Add an accessible SettingHelp tooltip (focusable button + aria-label) wrapped by
  a page-level TooltipProvider; used for the System + Reduce-motion explanations.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/components/ui/sonner.tsx

@@ -2,11 +2,11 @@ import { Toaster as Sonner } from 'sonner';
 import { useTheme } from '../../contexts/ThemeContext';
 
 export function Toaster() {
-  const { theme } = useTheme();
+  const { resolvedTheme } = useTheme();
 
   return (
     <Sonner
-      theme={theme}
+      theme={resolvedTheme}
       position="top-right"
       closeButton
       expand={false}

client/components/ui/theme-toggle.tsx

@@ -1,5 +1,5 @@
 import * as DropdownMenuPrimitive from '@radix-ui/react-dropdown-menu';
-import { Sun, Moon, type LucideIcon } from 'lucide-react';
+import { Sun, Moon, Monitor, type LucideIcon } from 'lucide-react';
 import type { ComponentProps } from 'react';
 import { cn } from '@/lib/utils';
 import { useTheme } from '@/contexts/ThemeContext';
@@ -56,6 +56,7 @@ interface ThemeOption {
 const THEMES: ThemeOption[] = [
   { value: 'light',  label: 'Light',  Icon: Sun },
   { value: 'dark',   label: 'Dark',   Icon: Moon },
+  { value: 'system', label: 'System', Icon: Monitor },
 ];
 
 /* ── ThemeToggle ────────────────────────────────────────────── */

client/contexts/MotionPreferenceContext.tsx

@@ -0,0 +1,55 @@
+import { createContext, useContext, useState, type ReactNode } from 'react';
+import { MotionConfig } from 'framer-motion';
+
+/**
+ * "Reduce motion" device preference, persisted to localStorage under
+ * 'bt-reduce-motion'. When ON, framer-motion animations are forced off
+ * (`reducedMotion="always"`); when OFF we pass "user" so the OS
+ * `prefers-reduced-motion` setting is still honored.
+ */
+
+const STORAGE_KEY = 'bt-reduce-motion';
+
+function loadPref(): boolean {
+  try {
+    return localStorage.getItem(STORAGE_KEY) === 'true';
+  } catch {
+    return false;
+  }
+}
+
+interface MotionPreferenceValue {
+  reduceMotion: boolean;
+  setReduceMotion: (v: boolean) => void;
+}
+
+const MotionPreferenceContext = createContext<MotionPreferenceValue | null>(null);
+
+export function MotionPreferenceProvider({ children }: { children: ReactNode }) {
+  const [reduceMotion, setReduceMotionState] = useState<boolean>(loadPref);
+
+  const setReduceMotion = (v: boolean) => {
+    setReduceMotionState(v);
+    try {
+      localStorage.setItem(STORAGE_KEY, String(v));
+    } catch {
+      // localStorage unavailable
+    }
+  };
+
+  return (
+    <MotionPreferenceContext.Provider value={{ reduceMotion, setReduceMotion }}>
+      <MotionConfig reducedMotion={reduceMotion ? 'always' : 'user'}>
+        {children}
+      </MotionConfig>
+    </MotionPreferenceContext.Provider>
+  );
+}
+
+export function useMotionPreference() {
+  const ctx = useContext(MotionPreferenceContext);
+  if (!ctx) {
+    throw new Error('useMotionPreference must be used within a MotionPreferenceProvider');
+  }
+  return ctx;
+}

client/contexts/ThemeContext.tsx

@@ -1,27 +1,43 @@
 import { createContext, useContext, useEffect, useState, type ReactNode } from 'react';
 
 /**
- * Themes: 'light' | 'dark'
+ * Themes: 'light' | 'dark' | 'system'
  * Persisted to localStorage under key 'bt-theme'.
  *
+ * 'system' follows the device's `prefers-color-scheme` and updates live when the
+ * OS flips. `resolvedTheme` is always the concrete 'light' | 'dark' actually
+ * applied — consume it for anything that needs a real value (e.g. Sonner).
+ *
  * Class mapping on document.documentElement:
- *   light       → (no classes)
- *   dark        → 'dark'
+ *   light → (no classes)   dark → 'dark'
  */
 
-type Theme = 'light' | 'dark';
+type Theme = 'light' | 'dark' | 'system';
+type ResolvedTheme = 'light' | 'dark';
 
 const STORAGE_KEY = 'bt-theme';
-const VALID_THEMES: Theme[] = ['light', 'dark'];
+const VALID_THEMES: Theme[] = ['light', 'dark', 'system'];
 const DEFAULT_THEME: Theme = 'dark';
 
-function applyTheme(theme: Theme) {
-  const root = document.documentElement;
-  root.classList.remove('dark');
-  if (theme === 'dark') {
-    root.classList.add('dark');
+// Keep in sync with the inline pre-bundle script in index.html.
+const THEME_COLORS: Record<ResolvedTheme, string> = { light: '#ffffff', dark: '#18181b' };
+
+function systemPrefersDark(): boolean {
+  return typeof window !== 'undefined' && !!window.matchMedia?.('(prefers-color-scheme: dark)').matches;
 }
-  // 'light' → no classes
+
+function resolveTheme(theme: Theme): ResolvedTheme {
+  if (theme === 'system') return systemPrefersDark() ? 'dark' : 'light';
+  return theme;
+}
+
+function applyTheme(theme: Theme): ResolvedTheme {
+  const resolved = resolveTheme(theme);
+  const root = document.documentElement;
+  root.classList.toggle('dark', resolved === 'dark');
+  const meta = document.querySelector('meta[name="theme-color"]');
+  if (meta) meta.setAttribute('content', THEME_COLORS[resolved]);
+  return resolved;
 }
 
 function loadStoredTheme(): Theme {
@@ -37,6 +53,7 @@ function loadStoredTheme(): Theme {
 
 interface ThemeContextValue {
   theme: Theme;
+  resolvedTheme: ResolvedTheme;
   setTheme: (theme: string) => void;
 }
 
@@ -45,29 +62,40 @@ const ThemeContext = createContext<ThemeContextValue | null>(null);
 export function ThemeProvider({ children }: { children: ReactNode }) {
   const [theme, setThemeState] = useState<Theme>(() => {
     const stored = loadStoredTheme();
-    // Apply immediately (before first paint) to avoid flash
+    // Apply immediately (before first paint) to avoid flash.
     applyTheme(stored);
     return stored;
   });
+  const [resolvedTheme, setResolvedTheme] = useState<ResolvedTheme>(() => resolveTheme(loadStoredTheme()));
 
   const setTheme = (newTheme: string) => {
     if (!(VALID_THEMES as string[]).includes(newTheme)) return;
-    applyTheme(newTheme as Theme);
-    setThemeState(newTheme as Theme);
+    const next = newTheme as Theme;
+    setResolvedTheme(applyTheme(next));
+    setThemeState(next);
     try {
-      localStorage.setItem(STORAGE_KEY, newTheme);
+      localStorage.setItem(STORAGE_KEY, next);
     } catch {
       // localStorage unavailable
     }
   };
 
-  // Keep DOM in sync if theme state ever changes externally
+  // Keep DOM in sync if theme state ever changes externally.
   useEffect(() => {
-    applyTheme(theme);
+    setResolvedTheme(applyTheme(theme));
+  }, [theme]);
+
+  // While on 'system', re-apply live when the OS light/dark preference flips.
+  useEffect(() => {
+    if (theme !== 'system' || typeof window === 'undefined' || !window.matchMedia) return;
+    const mq = window.matchMedia('(prefers-color-scheme: dark)');
+    const onChange = () => setResolvedTheme(applyTheme('system'));
+    mq.addEventListener('change', onChange);
+    return () => mq.removeEventListener('change', onChange);
   }, [theme]);
 
   return (
-    <ThemeContext.Provider value={{ theme, setTheme }}>
+    <ThemeContext.Provider value={{ theme, resolvedTheme, setTheme }}>
       {children}
     </ThemeContext.Provider>
   );

client/main.tsx

@@ -5,12 +5,14 @@ import App from './App';
 import { Toaster } from './components/ui/sonner';
 import { AuthProvider } from './hooks/useAuth';
 import { ThemeProvider } from './contexts/ThemeContext';
+import { MotionPreferenceProvider } from './contexts/MotionPreferenceContext';
 import 'sonner/dist/styles.css';
 import './index.css';
 
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
     <ThemeProvider>
+      <MotionPreferenceProvider>
         <BrowserRouter>
           <AuthProvider>
             <App />
@@ -19,6 +21,7 @@ ReactDOM.createRoot(document.getElementById('root')!).render(
           {/* Global shadcn/Sonner toast system */}
           <Toaster />
         </BrowserRouter>
+      </MotionPreferenceProvider>
     </ThemeProvider>
   </React.StrictMode>
 );

client/pages/SettingsPage.tsx

@@ -2,7 +2,7 @@ import { useState, useEffect, useCallback, type ComponentType, type ReactNode }
 import { useNavigate } from 'react-router-dom';
 import { useQueryClient } from '@tanstack/react-query';
 import { toast } from 'sonner';
-import { AlertCircle, Moon, RefreshCw, Sun, Users } from 'lucide-react';
+import { AlertCircle, Moon, Monitor, RefreshCw, Sun, Users, HelpCircle } from 'lucide-react';
 import { api } from '@/api';
 import { cn, errMessage, settingEnabled } from '@/lib/utils';
 import { Button } from '@/components/ui/button';
@@ -12,7 +12,9 @@ import {
   Select, SelectTrigger, SelectValue, SelectContent, SelectItem,
 } from '@/components/ui/select';
 import { Switch } from '@/components/ui/switch';
+import { Tooltip, TooltipContent, TooltipProvider, TooltipTrigger } from '@/components/ui/tooltip';
 import { useTheme } from '@/contexts/ThemeContext';
+import { useMotionPreference } from '@/contexts/MotionPreferenceContext';
 import { useAuth } from '@/hooks/useAuth';
 import { useAutoSave } from '@/hooks/useAutoSave';
 import { SaveStatus } from '@/components/ui/save-status';
@@ -79,6 +81,27 @@ function SettingRow({ label, description, children }: { label: ReactNode; descri
   );
 }
 
+// ─── Inline help tooltip ──────────────────────────────────────────────────────
+// A focusable help icon that explains a non-obvious setting. Sits next to a
+// SettingRow label. Keyboard + screen-reader accessible (button + aria-label).
+
+function SettingHelp({ text, label }: { text: string; label: string }) {
+  return (
+    <Tooltip>
+      <TooltipTrigger asChild>
+        <button
+          type="button"
+          aria-label={`${label}: help`}
+          className="ml-1.5 inline-flex h-4 w-4 shrink-0 items-center justify-center rounded-full text-muted-foreground/70 transition-colors hover:text-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring"
+        >
+          <HelpCircle className="h-3.5 w-3.5" />
+        </button>
+      </TooltipTrigger>
+      <TooltipContent className="max-w-[260px] text-xs leading-relaxed">{text}</TooltipContent>
+    </Tooltip>
+  );
+}
+
 // ─── Theme Card ───────────────────────────────────────────────────────────────
 
 function ThemeCard({ value, label, icon: Icon, currentTheme, onSelect }: {
@@ -111,15 +134,30 @@ function ThemeCard({ value, label, icon: Icon, currentTheme, onSelect }: {
 
 function AppearanceSection() {
   const { theme, setTheme } = useTheme();
+  const { reduceMotion, setReduceMotion } = useMotionPreference();
 
   return (
     <SectionCard title="Appearance">
-      <SettingRow label="Theme" description="Choose your preferred color scheme.">
+      <SettingRow
+        label={<span className="inline-flex items-center">Theme<SettingHelp label="Theme" text="“System” follows your device's light/dark setting and switches automatically when it changes." /></span>}
+        description="Choose your preferred color scheme."
+      >
         <div className="flex gap-2">
           <ThemeCard value="light"  label="Light"  icon={Sun}     currentTheme={theme} onSelect={setTheme} />
           <ThemeCard value="dark"   label="Dark"   icon={Moon}    currentTheme={theme} onSelect={setTheme} />
+          <ThemeCard value="system" label="System" icon={Monitor} currentTheme={theme} onSelect={setTheme} />
         </div>
       </SettingRow>
+      <SettingRow
+        label={<span className="inline-flex items-center">Reduce motion<SettingHelp label="Reduce motion" text="Turns off animations and transitions across the app. Also respects your device's reduced-motion setting." /></span>}
+        description="Minimize animations and transitions."
+      >
+        <Switch
+          checked={reduceMotion}
+          onCheckedChange={setReduceMotion}
+          aria-label="Reduce motion"
+        />
+      </SettingRow>
     </SectionCard>
   );
 }
@@ -380,6 +418,7 @@ export default function SettingsPage() {
   }
 
   return (
+    <TooltipProvider delayDuration={200}>
     <div>
 
       {/* Page header — flat on background, live save status on the right */}
@@ -545,5 +584,6 @@ export default function SettingsPage() {
       </div>
 
     </div>
+    </TooltipProvider>
   );
 }

index.html

@@ -11,6 +11,24 @@
   <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
   <meta name="apple-mobile-web-app-title" content="BillTracker">
   <title>Bill Tracker</title>
+  <script>
+    // Pre-bundle theme apply — avoids a light/dark flash. Mirrors applyTheme in
+    // client/contexts/ThemeContext.tsx (theme 'system' follows the OS).
+    (function () {
+      try {
+        var s = localStorage.getItem('bt-theme');
+        if (s === 'dark-purple') s = 'dark';
+        var theme = (s === 'light' || s === 'dark' || s === 'system') ? s : 'dark';
+        var resolved = theme === 'system'
+          ? ((window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) ? 'dark' : 'light')
+          : theme;
+        var root = document.documentElement;
+        if (resolved === 'dark') root.classList.add('dark'); else root.classList.remove('dark');
+        var m = document.querySelector('meta[name="theme-color"]');
+        if (m) m.setAttribute('content', resolved === 'dark' ? '#18181b' : '#ffffff');
+      } catch (e) { /* localStorage/matchMedia unavailable — keep default */ }
+    })();
+  </script>
 </head>
 <body>
   <div id="root"></div>