fix: resolve ESLint errors for React compiler rules

- Chat.tsx: replace mutable variable in render IIFE with useMemo+reduce
  to satisfy react-hooks/immutability rule
- ConnectionBanner.tsx: move setState calls into a useCallback to avoid
  synchronous setState in effect body (react-hooks/set-state-in-effect)
- useGateway.ts: suppress set-state-in-effect for legitimate mount init

src/hooks/useGateway.ts

@@ -307,6 +307,8 @@ export function useGateway() {
     initRef.current = true;
     const stored = getStoredCredentials();
     if (stored) {
+      // Init on mount — setupClient sets state as part of establishing the connection
+      // eslint-disable-next-line react-hooks/set-state-in-effect
       setupClient(stored.url, stored.token);
     } else {
       setAuthenticated(false);