fix: resolve back step, audio labels, progress parsing, file size pre-check, and stale job cleanup

- Fix handleBackStep to redirect to download prompt when step state is nil
  (previously silent no-op after container selection cleared the state)
- Improve formatLabel for audio-only entries without bitrate: use ID + extension
  instead of generic 'audio', fixing duplicate entries in format list
- Add yt-dlp --progress-template for reliable JSON progress parsing across all
  sites (fixes 0.0% progress on sites like xnxx.com where [download] lines
  are absent); keep regex parser as fallback for older yt-dlp versions
- Add file size pre-check in handleContainerSelection before download starts
  (previously checked only after download completed, wasting bandwidth)
- Clear stale active jobs and step state on /start for a clean slate

internal/dl/ytdlp_test.go

@@ -89,7 +89,8 @@ func TestFormatLabel(t *testing.T) {
 		want string
 	}{
 		{domain.Format{IsAudioOnly: true, Bitrate: "128k"}, "128k"},
-		{domain.Format{IsAudioOnly: true}, "audio"},
+		{domain.Format{ID: "140", Extension: "m4a", IsAudioOnly: true}, "140 (m4a)"},
+		{domain.Format{ID: "251", IsAudioOnly: true}, "251"},
 		{domain.Format{Resolution: "1920x1080", Height: 1080}, "1920x1080"},
 		{domain.Format{Height: 720}, "720p"},
 		{domain.Format{ID: "137"}, "137"},