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/hooks/useFeatureFlags.ts

@@ -6,7 +6,7 @@ import { contestsApi } from '@/api/contests';
 import { pollsApi } from '@/api/polls';
 
 export function useFeatureFlags() {
-  const { isAuthenticated } = useAuthStore();
+  const isAuthenticated = useAuthStore((state) => state.isAuthenticated);
 
   const { data: referralTerms } = useQuery({
     queryKey: ['referral-terms'],