🐛(frontend) reset video ref on track stop for state transitions

Reset video element reference when track stops to ensure "camera
starting" to "camera started" message transitions work correctly on
repeated camera toggles.

Previously only worked on initial video element load. Now properly
handles state transitions for multiple camera enable/disable cycles.

src/frontend/src/features/rooms/components/Join.tsx

@@ -161,7 +161,11 @@ export const Join = ({
 
     return () => {
       videoTrack?.detach()
-      videoElement?.removeEventListener('loadedmetadata', handleVideoLoaded)
+      if (videoElement) {
+        videoElement.removeEventListener('loadedmetadata', handleVideoLoaded)
+        videoElement.style.opacity = '0'
+      }
+      isVideoInitiated.current = false
     }
   }, [videoTrack, videoEnabled])