refactor(ts): convert client/lib/utils to TypeScript (TS4)

The highest-traffic leaf module (cn, fmt, date/byte/uptime formatters,
categoryColor) is now strict .ts. fmt inherits formatUSD's branded-dollars input
via Parameters<typeof formatUSD>; noUncheckedIndexedAccess handled (destructuring
defaults, in-range assertion). .jsx callers unaffected (Vite resolves the .ts).
typecheck + build + 48 client tests green.

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

client/lib/utils.ts

@@ -1,36 +1,37 @@
-import { clsx } from 'clsx';
+import { clsx, type ClassValue } from 'clsx';
 import { twMerge } from 'tailwind-merge';
 import { formatUSD } from './money';
 
-export function cn(...inputs) {
+export function cn(...inputs: ClassValue[]): string {
   return twMerge(clsx(inputs));
 }
 
 // Canonical dollar formatter for the app ("$1,234.50"). Kept here for the many
 // existing call sites; the implementation lives in ./money (formatUSD) so all
-// currency formatting has a single source of truth.
+// currency formatting has a single source of truth. Inherits formatUSD's typed
-export function fmt(amount) {
+// (branded-dollars) input.
+export function fmt(amount: Parameters<typeof formatUSD>[0]): string {
   return formatUSD(amount);
 }
 
-export function fmtDate(dateStr) {
+export function fmtDate(dateStr: string | null | undefined): string {
   if (!dateStr) return '—';
-  const [y, m, d] = dateStr.split('-');
+  const [y = '', m = '', d = ''] = dateStr.split('-');
   return `${parseInt(m)}/${parseInt(d)}/${y}`;
 }
 
-export function localDateString(date = new Date()) {
+export function localDateString(date: Date = new Date()): string {
   const year = date.getFullYear();
   const month = String(date.getMonth() + 1).padStart(2, '0');
   const day = String(date.getDate()).padStart(2, '0');
   return `${year}-${month}-${day}`;
 }
 
-export function todayStr() {
+export function todayStr(): string {
   return localDateString();
 }
 
-export function fmtUptime(seconds) {
+export function fmtUptime(seconds: number): string {
   const d = Math.floor(seconds / 86400);
   const h = Math.floor((seconds % 86400) / 3600);
   const m = Math.floor((seconds % 3600) / 60);
@@ -41,14 +42,21 @@ export function fmtUptime(seconds) {
   return `${s}s`;
 }
 
-export function fmtBytes(bytes) {
+export function fmtBytes(bytes: number | null | undefined): string {
   if (!bytes) return '0 B';
   if (bytes < 1024)    return `${bytes} B`;
   if (bytes < 1048576) return `${(bytes / 1024).toFixed(1)} KB`;
   return `${(bytes / 1048576).toFixed(2)} MB`;
 }
 
-const CATEGORY_COLOR_TONES = [
+interface CategoryTone {
+  border: string;
+  bg: string;
+  text: string;
+  bar: string;
+}
+
+const CATEGORY_COLOR_TONES: CategoryTone[] = [
   { border: 'border-emerald-300/50', bg: 'bg-emerald-400/15', text: 'text-emerald-700 dark:text-emerald-200', bar: 'bg-emerald-500' },
   { border: 'border-sky-300/50',     bg: 'bg-sky-400/15',     text: 'text-sky-700 dark:text-sky-200',     bar: 'bg-sky-500' },
   { border: 'border-amber-300/50',   bg: 'bg-amber-400/15',   text: 'text-amber-700 dark:text-amber-200',   bar: 'bg-amber-500' },
@@ -61,12 +69,12 @@ const CATEGORY_COLOR_TONES = [
 
 // Deterministic color tone for a category (or merchant) name, used for
 // badges and avatars so the same name always renders the same color.
-export function categoryColor(name) {
+export function categoryColor(name: string | null | undefined): CategoryTone {
   const key = String(name || 'Uncategorized');
   let hash = 0;
   for (let i = 0; i < key.length; i++) {
     hash = (hash * 31 + key.charCodeAt(i)) | 0;
   }
   const index = Math.abs(hash) % CATEGORY_COLOR_TONES.length;
-  return CATEGORY_COLOR_TONES[index];
+  return CATEGORY_COLOR_TONES[index]!; // index is always in range
 }