fix: react-doctor → 64/100 (Bugs 122, Perf 286, A11y 613, Maint 436)

- CartContext: lazy initializers replace mount-only useEffect
  hydration; remove 8 no-initialize-state warnings
- Toast/AdminSearchInput: React 19 useContext/use + drop
  forwardRef (3 no-react19-deprecated-apis)
- ProductFormModal: lazy initializers + useSyncExternalStore
  for mount; parent adds key=editingProduct.id
- InstallPrompt: useReducer for prompt state (no-cascading-set-state)
- QRScanModal: ref-based latest-callback pattern replaces
  useEffectEvent deps mistake
- OnboardingFlow: functional setState (rerender-functional-setstate)
- UsersPage/StopsCalendar/FeaturesAndStats: lazy initializers
  (rerender-lazy-state-init)
- FAQClientPage: server-side brand settings fetch via getBrandSettingsPublic
  in layout; remove supabase import
- LandingPageWrapper: href='#' → href='#top' (anchor-is-valid)
- TuxedoVideoHero: replace animate-bounce with ease-out-expo
  (no-inline-bounce-easing)
- ProductTableClient: useCallback for handleDeleted
  (jsx-no-new-function-as-prop)
- excel-parser: pre-compile delimiter regexes (js-hoist-regexp)
- water-log/settings: Promise.all for parallel DB calls
  (async-parallel)
- ToastNotification: extract toast store to separate file
  (only-export-components)
- WholesaleClient: inline <WholesaleIcon/> instead of hoisting to
  const (rendering-hoist-jsx)

src/app/login/LoginClient.tsx

@@ -23,7 +23,8 @@ export default function LoginClient({ error }: LoginClientProps) {
   const [localError, setLocalError] = useState<string | null>(null);
   const [, startDevLoginTransition] = useTransition();
 
-  async function handleSignIn() {
+  async function handleSignIn(e: React.FormEvent<HTMLFormElement>) {
+    e.preventDefault();
     if (!email || !password) {
       setLocalError("Email and password are required.");
       return;
@@ -141,10 +142,7 @@ export default function LoginClient({ error }: LoginClientProps) {
               </div>
 
               <form
-                onSubmit={(e) => {
-                  e.preventDefault();
-                  handleSignIn();
-                }}
+                onSubmit={handleSignIn}
                 className="space-y-5"
                 noValidate
               >