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/domain/types.go

@@ -134,10 +134,11 @@ type DownloadJob struct {
 
 // UserPreferences holds per-user default preferences.
 type UserPreferences struct {
-	DefaultMediaType MediaType
-	DefaultQuality   string
-	DefaultContainer string
-	DefaultLanguage  string
+	DefaultMediaType   MediaType
+	DefaultAudioFormat string
+	DefaultVideoFormat string
+	DefaultContainer   string
+	DefaultLanguage    string
 }
 
 // UserQuotas holds the quota usage and limits for a user.