feat(casting): complete all remaining TODOs

- Expose RemoteMediaClient from useCasting for advanced operations
- Implement episode fetching from Jellyfin API for TV shows
- Add next episode detection with countdown UI showing episode name
- Wire audio/subtitle track changes to RemoteMediaClient.setActiveTrackIds
- Wire playback speed to RemoteMediaClient.setPlaybackRate
- Add tap-to-seek functionality to progress bar
- Update segment skip buttons to use remoteMediaClient seek wrapper
- Create comprehensive AirPlay implementation documentation

All casting system features are now complete before PR submission.

hooks/useCasting.ts

@@ -287,6 +287,9 @@ export const useCasting = (item: BaseItemDto | null) => {
     isChromecastAvailable: true, // Always available via react-native-google-cast
     isAirPlayAvailable: Platform.OS === "ios",
 
+    // Raw clients (for advanced operations)
+    remoteMediaClient: client,
+
     // Controls
     play,
     pause,