Persist author cache to disk for instant channel info across restarts

Back the in-memory authorCache with a JSON file in ~/Library/Caches/AuthorCache/.
Disk is lazy-loaded on first lookup and saved asynchronously on each cache update.
Capped at 500 entries to prevent unbounded growth.

- Cache author data from video detail API responses (PlayerService, VideoInfoView)
- Replace ChannelView's private CachedChannelHeader with shared CachedChannelData
- Enrich author with cached avatar/subscriber count in VideoChannelRow, TVDetailsPanel, VideoInfoView

Yattee/Services/Player/PlayerService.swift

@@ -303,6 +303,7 @@ final class PlayerService {
 
                 // Update state with full video details and selected stream
                 state.setCurrentVideo(fullVideo, stream: selectedStream, audioStream: selectedAudioStream)
+                CachedChannelData.cacheAuthor(fullVideo.author)
                 state.videoDetailsState = .loaded
                 lockDurationIfNeeded(for: fullVideo, stream: selectedStream)