fix(ui): folds primitives for RouteError + PiP fullscreen button (native-cinny audit 5/N)

- RouteError: raw <div>/<h2>/<p>/<button> (sans-serif, raw px) -> folds
  Box/Text/Button with config tokens.
- CallEmbedProvider PiP fullscreen control: raw <button> with ⊡/⛶ glyphs ->
  folds IconButton reusing the exported FullscreenIcon/ExitFullscreenIcon SVGs
  from Controls (consistent with the main fullscreen button). The intentional
  dark over-video scrim is kept.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/call/Controls.tsx

@@ -166,13 +166,13 @@ export function ScreenShareButton({ enabled, onToggle }: ScreenShareButtonProps)
   );
 }
 
-const FullscreenIcon = () => (
+export 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 = () => (
+export 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>