perf: batch mobile keyboard viewport handlers with rAF and debounce (#660)

Reduce layout thrashing during virtual keyboard open/close on mobile by
batching resize and visualViewport event handlers to once per animation
frame. No visual or behavioral changes — desktop is unaffected.

- MainLayout: rAF-batch visualViewport resize/scroll and window resize
  listeners (was firing 10-20x per frame during keyboard animation)
- Header: rAF-batch ResizeObserver and resize/orientationchange callbacks
- ChatContainer: rAF-batch ResizeObserver and resize fallback callbacks
- useDeviceInfo: 150ms debounce on resize listener (device type does not
  change during keyboard transitions)

packages/ui/src/lib/device.ts

@@ -161,12 +161,19 @@ export function useDeviceInfo(): DeviceInfo {
   React.useEffect(() => {
     if (typeof window === 'undefined') return;
 
+    let debounceTimer: ReturnType<typeof setTimeout>;
     const handleResize = () => {
-      setDeviceInfo(getDeviceInfo());
+      clearTimeout(debounceTimer);
+      debounceTimer = setTimeout(() => {
+        setDeviceInfo(getDeviceInfo());
+      }, 150);
     };
 
     window.addEventListener('resize', handleResize);
-    return () => window.removeEventListener('resize', handleResize);
+    return () => {
+      clearTimeout(debounceTimer);
+      window.removeEventListener('resize', handleResize);
+    };
   }, []);
 
   React.useEffect(() => {