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/components/CallEmbedProvider.tsx

@@ -9,6 +9,7 @@ import {
   config,
   Dialog,
   Icon,
+  IconButton,
   Icons,
   Overlay,
   OverlayBackdrop,
@@ -51,6 +52,7 @@ import { mxcUrlToHttp, getMxIdLocalPart } from '../utils/matrix';
 import { RoomAvatar, RoomIcon } from './room-avatar';
 import { useRoomNavigate } from '../hooks/useRoomNavigate';
 import { getChatBg } from '../features/lotus/chatBackground';
+import { ExitFullscreenIcon, FullscreenIcon } from '../features/call/Controls';
 import { useTheme, ThemeKind } from '../hooks/useTheme';
 import { useSetting } from '../state/hooks/settings';
 import { settingsAtom } from '../state/settings';
@@ -1095,10 +1097,13 @@ export function CallEmbedProvider({ children }: CallEmbedProviderProps) {
             >
               <div style={{ display: 'flex', gap: config.space.S100, alignItems: 'center' }}>
                 {document.fullscreenEnabled && (
-                  <button
+                  <IconButton
                     type="button"
+                    size="300"
+                    radii="300"
+                    variant="Surface"
+                    fill="None"
                     aria-label={pipIsFullscreen ? 'Exit fullscreen' : 'Fullscreen camera'}
-                    title={pipIsFullscreen ? 'Exit fullscreen' : 'Fullscreen camera'}
                     onClick={(e) => {
                       e.stopPropagation();
                       handlePipFullscreen();
@@ -1107,19 +1112,11 @@ export function CallEmbedProvider({ children }: CallEmbedProviderProps) {
                       // Dark scrim is intentional for legibility over arbitrary video.
                       background: 'rgba(0,0,0,0.65)',
                       backdropFilter: 'blur(4px)',
-                      border: 'none',
-                      borderRadius: config.radii.R300,
-                      padding: `${config.space.S100} ${config.space.S200}`,
                       color: '#fff',
-                      fontSize: '13px',
-                      cursor: 'pointer',
-                      lineHeight: 1,
-                      display: 'flex',
-                      alignItems: 'center',
                     }}
                   >
-                    {pipIsFullscreen ? '⊡' : '⛶'}
-                  </button>
+                    {pipIsFullscreen ? <ExitFullscreenIcon /> : <FullscreenIcon />}
+                  </IconButton>
                 )}
                 <div
                   style={{