When PiP is active, the MPV render view shows a black frame since
rendering goes to the PiP sample buffer layer. Overlay the video
thumbnail (preferring DeArrow) on top to cover the black area,
fading it in/out smoothly when PiP starts/stops.
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
Reset isCommentsExpanded and commentsFrame on the NavigationCoordinator
directly when the portrait panel is dismissed, since PortraitDetailsPanel
owns its own @State that doesn't sync back through .onChange during dismiss.
Also track comments overlay frame via GeometryReader so the dismiss gesture
can allow swipes outside the comments area instead of blanket-blocking.
