Add watched tracking, resume positions, sort, density slider, channel metadata, mpv IPC, browser selector, auto-rescan

- SQLite DB for watched/position persistence (database.rs)
- Mark watched toggle on cards and detail panel
- mpv IPC socket integration for resume position tracking (unix)
- Sort by title, duration, file size
- Thumbnail density slider in top bar
- Channel metadata banner (subscriber count, uploader, channel URL)
- Cookie browser selector dropdown in settings
- Auto-rescan library when a download job finishes
- Fix yt-dlp --no-progress-bar flag (does not exist; removed)
- Browser field wired through config → downloader

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Cargo.lock

@@ -4820,6 +4820,7 @@ dependencies = [
  "image",
  "rusqlite",
  "serde",
+ "serde_json",
  "toml",
  "tray-icon",
 ]