refactor: rename Format to Quality, add container Audio/Video sub-pickers

- Rename 'Format' button to 'Quality' in settings (audio_quality/video_quality)
- Add DefaultAudioContainer and DefaultVideoContainer to UserPreferences
- Container settings now has Audio/Video sub-pickers (mp3/m4a/opus vs mp4/mkv/webm)
- Add migration 003 for new container columns
- Update repo layer and playlist flow for new fields
- Update back routing: back_quality, back_container

internal/repo/store_test.go

@@ -66,7 +66,8 @@ func TestPreferences(t *testing.T) {
 	prefs.DefaultMediaType = domain.MediaTypeAudio
 	prefs.DefaultAudioFormat = "best"
 	prefs.DefaultVideoFormat = "1080p"
-	prefs.DefaultContainer = "mp3"
+	prefs.DefaultAudioContainer = "mp3"
+	prefs.DefaultVideoContainer = "mp4"
 	prefs.DefaultLanguage = "en"
 	if err := s.UpdatePreferences(userID, prefs); err != nil {
 		t.Fatalf("UpdatePreferences() failed: %v", err)
@@ -85,8 +86,11 @@ func TestPreferences(t *testing.T) {
 	if got.DefaultVideoFormat != "1080p" {
 		t.Errorf("video format = %q, want 1080p", got.DefaultVideoFormat)
 	}
-	if got.DefaultContainer != "mp3" {
-		t.Errorf("container = %q, want mp3", got.DefaultContainer)
+	if got.DefaultAudioContainer != "mp3" {
+		t.Errorf("audio container = %q, want mp3", got.DefaultAudioContainer)
+	}
+	if got.DefaultVideoContainer != "mp4" {
+		t.Errorf("video container = %q, want mp4", got.DefaultVideoContainer)
 	}
 	if got.DefaultLanguage != "en" {
 		t.Errorf("language = %q, want en", got.DefaultLanguage)