feat: screenshare fullscreen button + pip spotlight, fix screenshare view

- Remove revert-to-grid logic that was overriding EC's natural screenshare
  spotlight, causing fullscreen to show user avatars instead of the screen
- Add fullscreen button to call controls (visible when screensharing) that
  requests fullscreen on the call embed container
- Add FullscreenButton component with enter/exit SVG icons to Controls.tsx
- PIP mode: sync setPipMode to CallControl; auto-enable spotlight when
  screenshare is active in pip so the screenshare fills the window
- Make useCallControlState accept undefined control for safe use in
  CallEmbedProvider
- Add package-lock.json to .gitignore (generated by local npm install)

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

src/app/features/call/CallControls.tsx

@@ -21,6 +21,7 @@ import * as css from './styles.css';
 import {
   ChatButton,
   ControlDivider,
+  FullscreenButton,
   MicrophoneButton,
   ScreenShareButton,
   SoundButton,
@@ -32,13 +33,16 @@ import { settingsAtom } from '../../state/settings';
 import { useResizeObserver } from '../../hooks/useResizeObserver';
 import { stopPropagation } from '../../utils/keyboard';
 import { AsyncStatus, useAsyncCallback } from '../../hooks/useAsyncCallback';
+import { useCallEmbedRef } from '../../hooks/useCallEmbed';
 
 type CallControlsProps = {
   callEmbed: CallEmbed;
 };
 export function CallControls({ callEmbed }: CallControlsProps) {
   const controlRef = useRef<HTMLDivElement>(null);
+  const callEmbedRef = useCallEmbedRef();
   const [compact, setCompact] = useState(document.body.clientWidth < 500);
+  const [isFullscreen, setIsFullscreen] = useState(false);
 
   useResizeObserver(
     useCallback(() => {
@@ -49,6 +53,20 @@ export function CallControls({ callEmbed }: CallControlsProps) {
     useCallback(() => controlRef.current, []),
   );
 
+  useEffect(() => {
+    const onFullscreenChange = () => setIsFullscreen(!!document.fullscreenElement);
+    document.addEventListener('fullscreenchange', onFullscreenChange);
+    return () => document.removeEventListener('fullscreenchange', onFullscreenChange);
+  }, []);
+
+  const handleFullscreen = useCallback(() => {
+    if (document.fullscreenElement) {
+      document.exitFullscreen();
+    } else {
+      callEmbedRef.current?.requestFullscreen();
+    }
+  }, [callEmbedRef]);
+
   const { microphone, video, sound, screenshare, spotlight } = useCallControlState(
     callEmbed.control,
   );
@@ -338,6 +356,9 @@ export function CallControls({ callEmbed }: CallControlsProps) {
                 screenshare ? callEmbed.control.toggleScreenshare() : setShareConfirm(true)
               }
             />
+            {screenshare && (
+              <FullscreenButton isFullscreen={isFullscreen} onToggle={handleFullscreen} />
+            )}
           </Box>
         </Box>
         {!compact && <ControlDivider />}