perf: add Zustand selectors to prevent cascading re-renders

Replace all bare useAuthStore(), useBlockingStore(), and
useSuccessNotification() calls with individual field selectors.
This prevents components from re-rendering when unrelated store
fields change.

- 21 useAuthStore usages across 20 files: individual selectors
  for 1-2 fields, useShallow for 3+ fields
- 5 useBlockingStore usages: individual selectors
- 2 useSuccessNotification usages: individual selectors

src/pages/OAuthCallback.tsx

@@ -26,7 +26,8 @@ export default function OAuthCallback() {
   const navigate = useNavigate();
   const [searchParams] = useSearchParams();
   const [error, setError] = useState('');
-  const { loginWithOAuth, isAuthenticated } = useAuthStore();
+  const loginWithOAuth = useAuthStore((state) => state.loginWithOAuth);
+  const isAuthenticated = useAuthStore((state) => state.isAuthenticated);
 
   useEffect(() => {
     if (isAuthenticated) {