Remove 32 non-dotted keys (16 unused format specifiers, 16 word keys)
and replace with properly namespaced dotted keys following the existing
convention (common.*, player.*, search.*).
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
