refactor: redesign format selection with unified list and independent audio/video picking

- Replace type/quality selection flow with unified format list (video+audio combined)
- Add independent audio/video format picking: video-only formats prompt for audio track,
  audio-only formats offer optional video addition (skip when content is audio-only)
- Fix formatKeyboard to use format IDs as callback data instead of display labels
- Remove dead mediaTypeKeyboard function
- Fix double-close panic in readProgress (caller is sole owner of updates channel)
- Add FormatString field to DownloadJob for composite format specs (e.g. '137+140')
- Change quota calculation from sum of all format sizes to max single format size
- Fix 'Delete all data' button to use danger style with proper text
- Clean up routePrefixedCallback: remove type_/quality_, add format_/secondary_ routes
- Fix back navigation to rebuild format IDs list

internal/domain/types.go

@@ -117,6 +117,7 @@ type DownloadJob struct {
 	URL            string
 	MediaType      MediaType
 	SelectedFormat *Format
+	FormatString   string
 	Container      string
 	Language       string
 	Status         DownloadStatus