fix: resolve CI lint errors in Sidebar, TypingIndicator, and useGateway

- Sidebar: replace useEffect setState with callback pattern for filter reset
- TypingIndicator: initialize useRef with 0 instead of impure Date.now()
- TypingIndicator: remove redundant setElapsed(0) from mount effect
- useGateway: remove unused eslint-disable directive

src/components/TypingIndicator.tsx

@@ -12,11 +12,10 @@ function formatElapsed(seconds: number): string {
 export function TypingIndicator() {
   const t = useT();
   const [elapsed, setElapsed] = useState(0);
-  const startRef = useRef(Date.now());
+  const startRef = useRef(0);
 
   useEffect(() => {
     startRef.current = Date.now();
-    setElapsed(0);
     const interval = setInterval(() => {
       setElapsed(Math.floor((Date.now() - startRef.current) / 1000));
     }, 1000);