refactor: settings format picker redesign with Audio/Video sub-pickers, fix delete routing

- Replace Type/Quality settings with single Format button -> Audio/Video sub-pickers
- Each sub-picker shows format options (bitrates for audio, resolutions for video)
- Replace DefaultQuality with DefaultAudioFormat/DefaultVideoFormat in UserPreferences
- Add DB migration 002 for new preference columns
- Update repo layer SQL queries for new schema
- Update playlist flow to use new preference fields
- Fix missing deleteaccount and delaccount_ callback routing
- Add back_format routing for format picker navigation

internal/repo/migrations/002_preferences_format.sql

@@ -0,0 +1,7 @@
+-- +goose Up
+ALTER TABLE user_preferences ADD COLUMN default_audio_format TEXT NOT NULL DEFAULT 'best';
+ALTER TABLE user_preferences ADD COLUMN default_video_format TEXT NOT NULL DEFAULT 'best';
+
+-- +goose Down
+ALTER TABLE user_preferences DROP COLUMN default_audio_format;
+ALTER TABLE user_preferences DROP COLUMN default_video_format;