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/Controls.tsx

@@ -158,6 +158,56 @@ export function ScreenShareButton({ enabled, onToggle }: ScreenShareButtonProps)
   );
 }
 
+const FullscreenIcon = () => (
+  <svg width="1em" height="1em" viewBox="0 0 24 24" fill="currentColor">
+    <path d="M3 3h6v2H5v4H3V3zm12 0h6v6h-2V5h-4V3zM3 15h2v4h4v2H3v-6zm16 4h-4v2h6v-6h-2v4z" />
+  </svg>
+);
+
+const ExitFullscreenIcon = () => (
+  <svg width="1em" height="1em" viewBox="0 0 24 24" fill="currentColor">
+    <path d="M9 3H7v4H3v2h6V3zm6 0v6h6V7h-4V3h-2zM3 13v2h4v4h2v-6H3zm14 4v-4h2v6h-6v-2h4z" />
+  </svg>
+);
+
+type FullscreenButtonProps = {
+  isFullscreen: boolean;
+  onToggle: () => void;
+};
+export function FullscreenButton({ isFullscreen, onToggle }: FullscreenButtonProps) {
+  return (
+    <TooltipProvider
+      position="Top"
+      delay={500}
+      tooltip={
+        <Tooltip>
+          <Text size="T200">{isFullscreen ? 'Exit Fullscreen' : 'Fullscreen'}</Text>
+        </Tooltip>
+      }
+    >
+      {(anchorRef) => (
+        <IconButton
+          ref={anchorRef}
+          variant="Surface"
+          fill="Soft"
+          radii="400"
+          size="400"
+          onClick={onToggle}
+          aria-label={isFullscreen ? 'Exit Fullscreen' : 'Fullscreen'}
+          aria-pressed={isFullscreen}
+          outlined
+        >
+          {isFullscreen ? (
+            <ExitFullscreenIcon />
+          ) : (
+            <FullscreenIcon />
+          )}
+        </IconButton>
+      )}
+    </TooltipProvider>
+  );
+}
+
 export function ChatButton() {
   const [chat, setChat] = useAtom(callChatAtom);