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/CallControl.ts

@@ -16,6 +16,8 @@ export class CallControl extends EventEmitter implements CallControlState {
 
   private controlMutationObserver: MutationObserver;
 
+  private _pipMode = false;
+
   private get document(): Document | undefined {
     return this.iframe.contentDocument ?? this.iframe.contentWindow?.document;
   }
@@ -183,12 +185,6 @@ export class CallControl extends EventEmitter implements CallControlState {
     );
     this.emitStateUpdate();
 
-    // EC auto-switches to spotlight when screenshare starts — revert to grid
-    if (!prevScreenshare && screenshare) {
-      setTimeout(() => {
-        if (this.spotlight) this.gridButton?.click();
-      }, 600);
-    }
   }
 
   public setMicrophone(enabled: boolean) {
@@ -247,6 +243,10 @@ export class CallControl extends EventEmitter implements CallControlState {
     this.spotlightButton?.click();
   }
 
+  public setPipMode(pip: boolean) {
+    this._pipMode = pip;
+  }
+
   public toggleReactions() {
     this.reactionsButton?.click();
   }