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.toml

@@ -9,6 +9,7 @@ eframe = "0.29"
 image = { version = "0.25", default-features = false, features = ["webp", "jpeg", "png"] }
 toml = "0.8"
 serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
 rusqlite = { version = "0.31", features = ["bundled"] }
 tray-icon = "0.13"