feat(quick-7): add PinDefault config field with backend getter/setter

- Add PinDefault bool to favorites.Config struct with TOML tag
- Add GetPinDefaultPlaylist() and SetPinDefaultPlaylist() methods
- Include PinDefault in emitFavoritesChanged event payload
- Default to true (pinned) for new config installations

backend/favorites/config.go

@@ -33,6 +33,7 @@ const DefaultPlaylistName = "Favorites"
 type Config struct {
 	PlaylistID int64     `toml:"PlaylistID"`
 	IconStyle  IconStyle `toml:"IconStyle"`
+	PinDefault bool      `toml:"PinDefault"`
 }
 
 // ApplyDefaults fills zero-value fields with sensible defaults.