Adds a "Proxy videos" toggle in instance settings that routes video
streams through the instance instead of connecting directly to YouTube
CDN. Includes auto-detection of 403 blocks and live re-application of
proxy settings without requiring app restart or video reload.
The filter strip was passing the Invidious instance URL as serverURL to
AvatarURLBuilder, which built a Yattee Server-style /avatar/ path that
doesn't exist on Invidious. Now passes the actual Yattee Server URL
(matching SubscriptionsView pattern) and enriches channels from
CachedChannelData as a fallback when the API doesn't return thumbnails.
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
