fix(web): resolve the 5 actionable React-compiler lint advisories

Web lint warnings reduced from 20 → 15 by fixing the categories that
flag real architectural smells rather than the canonical
fetch-on-mount setState pattern.

Real fixes:

1. web/src/lib/use-theme.ts — replace useEffect + setState mount-sync
   pattern with React.useSyncExternalStore. The hook now subscribes to
   browser storage events, returns a stable snapshot for SSR, and uses
   a manual storage-event dispatch so same-document setters refresh
   correctly. Eliminates the cascading-render advisory and gains free
   cross-tab theme sync.

2. web/src/lib/use-keyboard-shortcuts.ts — move ref assignment from
   render time into a useEffect. Fixes the 'Cannot access refs during
   render' advisory without behavior change.

3. web/src/components/NoteEditor.tsx — move onSaveRef.current = onSave
   from render time into a useEffect for the same reason.

4. web/src/app/(app)/reviews/page.tsx — wrap handleDecision and
   handleBatchDecision in useCallback so the useEffect that depends
   on them no longer re-subscribes the keydown listener on every
   render. Fixes both react-hooks/exhaustive-deps warnings and the
   underlying perf bug they pointed at.

5. web/src/app/(app)/prompts/page.tsx — wrap loadTemplates in
   useCallback declared before the useEffect that calls it. Fixes
   the 'Cannot access variable before it is declared' advisory.

Remaining 15 warnings are React-compiler runtime hints about
fetchData().then(setData) patterns inside useEffect, which is the
canonical fetch-on-mount pattern shown in React's own docs. Resolving
them properly requires Suspense + use() or risky startTransition
wraps; both are out of scope and tracked under future tech debt.

Verified:
- pnpm --filter @notelett/web run typecheck: passes
- pnpm --filter @notelett/web run lint: 0 errors, 15 warnings (down 5)
- pnpm run verify: backend 380/380, web 96/96, mobile 97/97

web/src/app/(app)/prompts/page.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useEffect, useState } from "react";
+import { useCallback, useEffect, useState } from "react";
 import { Sparkles, FileText, Image, Layers, Trash2 } from "lucide-react";
 import { AppShell } from "@/components/AppShell";
 import { Badge, Button, Card } from "@/components/ui/Primitives";
@@ -28,11 +28,7 @@ export default function PromptsPage() {
   const [activeCategory, setActiveCategory] = useState<PromptCategory | "all">("all");
   const [loading, setLoading] = useState(true);
 
-  useEffect(() => {
-    void loadTemplates();
-  }, []);
-
-  async function loadTemplates() {
+  const loadTemplates = useCallback(async () => {
     setLoading(true);
     try {
       setTemplates(await listPromptTemplates());
@@ -41,7 +37,11 @@ export default function PromptsPage() {
     } finally {
       setLoading(false);
     }
-  }
+  }, []);
+
+  useEffect(() => {
+    void loadTemplates();
+  }, [loadTemplates]);
 
   async function handleDelete(id: string) {
     if (!confirm("Delete this custom template?")) return;

web/src/app/(app)/reviews/page.tsx

@@ -105,7 +105,7 @@ export default function ReviewsPage() {
     },
   ] as const;
 
-  async function handleDecision(decision: "approved" | "rejected") {
+  const handleDecision = useCallback(async (decision: "approved" | "rejected") => {
     if (!featuredProposal) {
       return;
     }
@@ -146,9 +146,9 @@ export default function ReviewsPage() {
     } finally {
       setIsSubmitting(false);
     }
-  }
+  }, [featuredProposal, reviewNote]);
 
-  async function handleBatchDecision(decision: "approved" | "rejected") {
+  const handleBatchDecision = useCallback(async (decision: "approved" | "rejected") => {
     const batchItems = approvalQueue.filter((item) => selectedBatchIds.has(item.id));
     if (batchItems.length === 0) return;
 
@@ -184,7 +184,7 @@ export default function ReviewsPage() {
     } finally {
       setIsSubmitting(false);
     }
-  }
+  }, [approvalQueue, selectedBatchIds, reviewNote]);
 
   useEffect(() => {
     function selectRelativeQueueItem(offset: number) {

web/src/components/NoteEditor.tsx

@@ -57,7 +57,14 @@ export function NoteEditor({
   const [explainResult, setExplainResult] = useState<string | null>(null);
   const toneMenuRef = useRef<HTMLDivElement>(null);
   const onSaveRef = useRef(onSave);
-  onSaveRef.current = onSave;
+
+  // Keep the latest onSave handler in a ref so the debounced auto-save
+  // effect always invokes the current closure. Assigning inside an effect
+  // (rather than during render) avoids the React 19 compiler warning
+  // about mutating refs during render.
+  useEffect(() => {
+    onSaveRef.current = onSave;
+  }, [onSave]);
 
   useEffect(() => {
     if (!toneMenuOpen) return;

web/src/lib/use-keyboard-shortcuts.ts

@@ -13,7 +13,13 @@ export interface KeyboardShortcut {
 
 export function useKeyboardShortcuts(shortcuts: KeyboardShortcut[]) {
   const shortcutsRef = useRef(shortcuts);
-  shortcutsRef.current = shortcuts;
+
+  // Keep the ref pointed at the latest shortcuts array. Assigning inside
+  // an effect (rather than during render) avoids the React 19 compiler
+  // warning about mutating refs during render.
+  useEffect(() => {
+    shortcutsRef.current = shortcuts;
+  }, [shortcuts]);
 
   useEffect(() => {
     function handleKeyDown(event: KeyboardEvent) {

web/src/lib/use-theme.ts

@@ -1,25 +1,47 @@
 'use client';
 
-import { useState, useEffect, useCallback } from 'react';
+import { useCallback, useEffect, useSyncExternalStore } from 'react';
 import { PRODUCT_ID } from '@/lib/product-config';
 
 type Theme = 'dark' | 'light';
 const STORAGE_KEY = `${PRODUCT_ID}_theme`;
 
-export function useTheme() {
-  const [theme, setThemeState] = useState<Theme>('dark');
+function readTheme(): Theme {
+  if (typeof window === 'undefined') return 'dark';
+  const stored = window.localStorage.getItem(STORAGE_KEY) as Theme | null;
+  return stored ?? 'dark';
+}
 
+function getServerSnapshot(): Theme {
+  return 'dark';
+}
+
+// Subscribe to storage events so the theme stays in sync across tabs.
+// React.useSyncExternalStore handles SSR + hydration correctly and avoids
+// the setState-in-effect pattern that the React 19 compiler warns about.
+function subscribe(callback: () => void) {
+  if (typeof window === 'undefined') return () => {};
+  window.addEventListener('storage', callback);
+  return () => window.removeEventListener('storage', callback);
+}
+
+export function useTheme() {
+  const theme = useSyncExternalStore(subscribe, readTheme, getServerSnapshot);
+
+  // Reflect the resolved theme on the <html> element after hydration so
+  // initial mount picks up the value from localStorage. Pure DOM mutation,
+  // no setState, so the React-compiler advisory does not trigger.
   useEffect(() => {
-    const stored = localStorage.getItem(STORAGE_KEY) as Theme | null;
-    const initial = stored ?? 'dark';
-    setThemeState(initial);
-    document.documentElement.setAttribute('data-theme', initial);
-  }, []);
+    document.documentElement.setAttribute('data-theme', theme);
+  }, [theme]);
 
   const setTheme = useCallback((t: Theme) => {
-    setThemeState(t);
-    localStorage.setItem(STORAGE_KEY, t);
+    if (typeof window === 'undefined') return;
+    window.localStorage.setItem(STORAGE_KEY, t);
     document.documentElement.setAttribute('data-theme', t);
+    // Manually dispatch a storage event so the current document re-reads
+    // the snapshot (browsers only fire storage events in OTHER tabs).
+    window.dispatchEvent(new StorageEvent('storage', { key: STORAGE_KEY, newValue: t }));
   }, []);
 
   const toggle = useCallback(() => {