channel URLs

A channel URL opens its page in the Browser so you see its videos first (handpick if you want). It does not auto-import. From there you can do either or both:

• Follow it in the Browser so its uploads join your feed. This already exists; the fix is that pasting a channel URL should land on its page, not silently subscribe with no feedback.
• Convert it into a lore playlist, pulling its videos into your library as a collection (videos arrive not-downloaded; download what you want). This is the original ask here.

Either way it remembers the source URL so it can re-sync. Playlists are the same shape (see #43).

Spans several layers: URL routing, the channel page as the entry point, source-linked re-sync. Not a small change.