fix: prefer-arrow-callback ESLint rule and Prettier formatting

Convert TypingIndicator named function expression to arrow function to
satisfy prefer-arrow-callback lint rule. Auto-format RoomInput.tsx to
resolve Prettier check failures in CI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/room/RoomInput.tsx

@@ -151,7 +151,9 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
     const creators = useRoomCreators(room);
 
     const [charCount, setCharCount] = useState(0);
-    useEffect(() => { setCharCount(0); }, [roomId]);
+    useEffect(() => {
+      setCharCount(0);
+    }, [roomId]);
 
     const alive = useAlive();
     const [msgDraft, setMsgDraft] = useAtom(roomIdToMsgDraftAtomFamily(roomId));