feat(tracker): prefetch adjacent month on nav hover for instant switching (P2)

usePrefetchTracker() warms the ['tracker', y, m] cache when the user hovers/
focuses the prev/next month buttons, so clicking is instant (no round-trip).
No-op if already cached and fresh.

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

client/hooks/useQueries.js

@@ -68,6 +68,19 @@ export function useInvalidateTrackerData() {
   }, [queryClient]);
 }
 
+// Prefetch a tracker month into the cache (e.g. on month-nav hover) so switching
+// to it is instant. No-op if it's already cached and fresh.
+export function usePrefetchTracker() {
+  const queryClient = useQueryClient();
+  return useCallback((year, month) => {
+    queryClient.prefetchQuery({
+      queryKey: ['tracker', year, month],
+      queryFn: () => api.tracker(year, month),
+      staleTime: 1000 * 60 * 5,
+    });
+  }, [queryClient]);
+}
+
 // ── Page data (migrated off manual useEffect + load()) ───────────────────────
 // The queryKey encodes the params, so React Query handles caching, request
 // dedup, cancellation, and out-of-order responses — no manual sequence guards.

client/pages/TrackerPage.jsx

@@ -3,7 +3,7 @@ import { useSearchParams } from 'react-router-dom';
 import { ChevronLeft, ChevronRight, AlertCircle, CheckCircle2, Plus, Search, RefreshCw, Landmark, ArrowUpToLine, ArrowUp, ArrowDown, BellOff, EyeOff, Settings2 } from 'lucide-react';
 import { toast } from 'sonner';
 import { api } from '@/api.js';
-import { useTracker, useDriftReport, useInvalidateTrackerData } from '@/hooks/useQueries';
+import { useTracker, useDriftReport, useInvalidateTrackerData, usePrefetchTracker } from '@/hooks/useQueries';
 import { useSearchPanelPreference } from '@/hooks/useSearchPanelPreference';
 import BillModal from '@/components/BillModal';
 import { makeBillDraft } from '@/lib/billDrafts';
@@ -301,12 +301,23 @@ export default function TrackerPage() {
     return () => window.removeEventListener('tracker:late-attributions', handler);
   }, []);
 
-  function navigate(delta) {
+  function adjacentMonth(delta) {
     let nm = month + delta;
     let ny = year;
     if (nm > 12) { ny += 1; nm = 1; }
     if (nm < 1)  { ny -= 1; nm = 12; }
-    updateParams({ year: ny, month: nm });
+    return { year: ny, month: nm };
+  }
+
+  function navigate(delta) {
+    updateParams(adjacentMonth(delta));
+  }
+
+  // Prefetch the adjacent month on hover/focus so clicking prev/next is instant.
+  const prefetchTracker = usePrefetchTracker();
+  function prefetchAdjacent(delta) {
+    const { year: ny, month: nm } = adjacentMonth(delta);
+    prefetchTracker(ny, nm);
   }
 
   function bankSyncMessage(result) {
@@ -651,6 +662,8 @@ export default function TrackerPage() {
             <Button
               size="icon" variant="ghost"
               onClick={() => navigate(-1)}
+              onMouseEnter={() => prefetchAdjacent(-1)}
+              onFocus={() => prefetchAdjacent(-1)}
               className="h-7 w-7 hover:bg-white/5"
               aria-label="Previous month"
             >
@@ -662,6 +675,8 @@ export default function TrackerPage() {
             <Button
               size="icon" variant="ghost"
               onClick={() => navigate(1)}
+              onMouseEnter={() => prefetchAdjacent(1)}
+              onFocus={() => prefetchAdjacent(1)}
               className="h-7 w-7 hover:bg-white/5"
               aria-label="Next month"
             >