🐛(frontend) unmount PiP portal synchronously on pagehide

When the PiP window closes, the browser destroys its document right
after `pagehide`. If the portal unmount is left to React's async
scheduling, it commits against a dead document and `removeChild`
throws "NotFoundError", crashing the app.

Subscribe `PictureInPicturePortal` to the Valtio store with
`sync: true`, and use `flushSync` in `usePictureInPicture` on
teardown so React unmounts the portal while the PiP document is
still alive.

Fix 019f42cf-86a9-7ad2-8e64-81b004ddc5de

src/frontend/src/features/pip/components/PictureInPicturePortal.tsx

@@ -7,7 +7,9 @@ import { useEffect, useMemo } from 'react'
 import { CrossDocumentOverlaysContext } from '@/primitives/CrossDocumentOverlaysContext'
 
 const InternalPortal = ({ children }: { children: React.ReactNode }) => {
-  const pipStoreSnap = useSnapshot(documentPictureInPictureStore)
+  const pipStoreSnap = useSnapshot(documentPictureInPictureStore, {
+    sync: true,
+  })
 
   const container = useMemo(() => {
     return pipStoreSnap?.window?.document.getElementById('root')
@@ -19,7 +21,7 @@ const InternalPortal = ({ children }: { children: React.ReactNode }) => {
     }
   }, [])
 
-  if (!container) return null
+  if (!container || !container.isConnected) return null
 
   return createPortal(
     /**

src/frontend/src/features/pip/hooks/usePictureInPicture.ts

@@ -1,5 +1,6 @@
 import { ref, useSnapshot } from 'valtio'
 import { useCallback, useMemo } from 'react'
+import { flushSync } from 'react-dom'
 import { documentPictureInPictureStore } from '@/stores/documentPictureInPicture'
 import { useTranslation } from 'react-i18next'
 
@@ -73,7 +74,9 @@ export const usePictureInPicture = () => {
 
         const cleanUp = () => {
           if (documentPictureInPictureStore.window === pipWindow) {
-            documentPictureInPictureStore.window = null
+            flushSync(() => {
+              documentPictureInPictureStore.window = null
+            })
           }
         }
         pipWindow.addEventListener('pagehide', () => cleanUp(), { once: true })