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/plugins/call/hooks.ts

@@ -32,13 +32,18 @@ export const useSendClientWidgetApiAction = (api: ClientWidgetApi) => {
   return sendWidgetAction;
 };
 
-export const useCallControlState = (control: CallControl): CallControlState => {
-  const [state, setState] = useState(control.getState());
+const DEFAULT_CONTROL_STATE = new CallControlState(false, false, false);
+
+export const useCallControlState = (control: CallControl | undefined): CallControlState => {
+  const [state, setState] = useState(control?.getState() ?? DEFAULT_CONTROL_STATE);
 
   useEffect(() => {
-    const handleUpdate = () => {
-      setState(control.getState());
-    };
+    if (!control) {
+      setState(DEFAULT_CONTROL_STATE);
+      return;
+    }
+    setState(control.getState());
+    const handleUpdate = () => setState(control.getState());
     control.on(CallControlEvent.StateUpdate, handleUpdate);
     return () => {
       control.off(CallControlEvent.StateUpdate, handleUpdate);