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/migrations/003_preferences_container.sql

@@ -0,0 +1,7 @@
+-- +goose Up
+ALTER TABLE user_preferences ADD COLUMN default_audio_container TEXT NOT NULL DEFAULT 'mp3';
+ALTER TABLE user_preferences ADD COLUMN default_video_container TEXT NOT NULL DEFAULT 'mp4';
+
+-- +goose Down
+ALTER TABLE user_preferences DROP COLUMN default_audio_container;
+ALTER TABLE user_preferences DROP COLUMN default_video_container;