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/components/blocking/BlacklistedScreen.tsx

@@ -3,7 +3,7 @@ import { useBlockingStore } from '../../store/blocking';
 
 export default function BlacklistedScreen() {
   const { t } = useTranslation();
-  const { blacklistedInfo } = useBlockingStore();
+  const blacklistedInfo = useBlockingStore((state) => state.blacklistedInfo);
 
   return (
     <div className="fixed inset-0 z-[100] flex flex-col items-center justify-center bg-dark-950 p-6">

src/components/blocking/ChannelSubscriptionScreen.tsx

@@ -7,7 +7,8 @@ const CHECK_COOLDOWN_SECONDS = 5;
 
 export default function ChannelSubscriptionScreen() {
   const { t } = useTranslation();
-  const { channelInfo, clearBlocking } = useBlockingStore();
+  const channelInfo = useBlockingStore((state) => state.channelInfo);
+  const clearBlocking = useBlockingStore((state) => state.clearBlocking);
   const [isChecking, setIsChecking] = useState(false);
   const [cooldown, setCooldown] = useState(0);
   const [error, setError] = useState<string | null>(null);

src/components/blocking/MaintenanceScreen.tsx

@@ -3,7 +3,7 @@ import { useBlockingStore } from '../../store/blocking';
 
 export default function MaintenanceScreen() {
   const { t } = useTranslation();
-  const { maintenanceInfo } = useBlockingStore();
+  const maintenanceInfo = useBlockingStore((state) => state.maintenanceInfo);
 
   return (
     <div className="fixed inset-0 z-[100] flex flex-col items-center justify-center bg-dark-950 p-6">