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>

src/config.rs

@@ -15,10 +15,18 @@ pub struct BackupSection {
     pub directory: PathBuf,
 }
 
-#[derive(Debug, Serialize, Deserialize, Clone, Default)]
+#[derive(Debug, Serialize, Deserialize, Clone)]
 pub struct PlayerSection {
     #[serde(default = "default_player")]
     pub command: String,
+    #[serde(default = "default_browser")]
+    pub browser: String,
+}
+
+impl Default for PlayerSection {
+    fn default() -> Self {
+        Self { command: default_player(), browser: default_browser() }
+    }
 }
 
 #[derive(Debug, Serialize, Deserialize, Clone)]
@@ -33,13 +41,9 @@ impl Default for UiSection {
     }
 }
 
-fn default_player() -> String {
-    "mpv".to_string()
-}
-
-fn default_theme() -> String {
-    "dark".to_string()
-}
+fn default_player() -> String { "mpv".to_string() }
+fn default_browser() -> String { "firefox".to_string() }
+fn default_theme() -> String { "dark".to_string() }
 
 impl Config {
     pub fn load(path: &Path) -> Result<Self, Box<dyn std::error::Error>> {