chore(deps): upgrade React 18.3 -> 19.2.7

react / react-dom 18.3.1 -> 19.2.7; @types/react 18.3.31 -> 19.2.17;
@types/react-dom 18.3.7 -> 19.2.3 (latest stable 19.2.x).

Only breaking change hitting our code: React 19 removed the no-arg useRef<T>()
overload. AppShell's logoutTimer -> useRef<T | undefined>(undefined) — the same
transform the official `types-react-codemod preset-19` (useRef-required-initial)
applies. No other preset-19 transforms were relevant (already on createRoot; no
ReactDOM.render, string refs, propTypes/defaultProps on function components).

No Next.js / RSC in this project (it's a Vite SPA served by Fastify), so those
React 19 concerns don't apply here. MUI v7, framer-motion 12, TanStack Query 5,
react-router-dom 6 and react-quill-new all installed cleanly against React 19
(no peer-dependency conflicts).

Gates: tsc 0 | build 0 | vitest 247/247 | eslint 0 errors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/ui/AppShell.tsx

@@ -19,7 +19,9 @@ export default function AppShell() {
   const [mobileOpen, setMobileOpen] = useState(false);
   const [loggingOut, setLoggingOut] = useState(false);
   const location = useLocation();
-  const logoutTimer = useRef<ReturnType<typeof setTimeout>>();
+  const logoutTimer = useRef<ReturnType<typeof setTimeout> | undefined>(
+    undefined,
+  );
 
   const handleLogout = useCallback(() => {
     setLoggingOut(true);