refactor: remove container selection, extract helpers, add logging and tests

- Remove container picker UI, keyboard, settings, and ContainerOptions
  entirely — yt-dlp handles container format automatically
- Remove Container field from DownloadJob struct and related references
- Extract buildFormatList helper to deduplicate format list building
  between handleURLInput and handleBackStep
- Break runDownload into trackDownloadProgress + handleDownloadCompletion
- Add HTTP timeout (30s) to downloadFile helper
- Log applyQuota and AddHistory errors instead of discarding
- Log nil stepState warnings in all handler entry points
- Include job.StderrLog in download failure messages
- Add tests for buildFormatList, findFormatByID, determineMediaType,
  and buildFormatString
- Update README with search, DRM fallback, and quota feature docs
- Remove unused strings import from keyboard.go

internal/handler/playlist.go

@@ -141,19 +141,15 @@ func (h *Handler) handlePlaylistConfirm(ctx context.Context, chatID int64, msgID
 		prefs, prefErr := h.Repo.GetOrCreatePreferences(userID)
 		if prefErr != nil {
 			prefs = &domain.UserPreferences{
-				DefaultMediaType:      domain.MediaTypeVideoAudio,
-				DefaultAudioFormat:    "best",
-				DefaultVideoFormat:    "best",
-				DefaultAudioContainer: "mp3",
-				DefaultVideoContainer: "mp4",
+				DefaultMediaType:   domain.MediaTypeVideoAudio,
+				DefaultAudioFormat: "best",
+				DefaultVideoFormat: "best",
 			}
 		}
 
 		formatID := prefs.DefaultVideoFormat
-		container := prefs.DefaultVideoContainer
 		if prefs.DefaultMediaType == domain.MediaTypeAudio {
 			formatID = prefs.DefaultAudioFormat
-			container = prefs.DefaultAudioContainer
 		}
 		format := &domain.Format{ID: formatID}
 		for _, f := range info.Formats {
@@ -172,7 +168,6 @@ func (h *Handler) handlePlaylistConfirm(ctx context.Context, chatID int64, msgID
 			URL:            entry.URL,
 			MediaType:      prefs.DefaultMediaType,
 			SelectedFormat: format,
-			Container:      container,
 			Language:       prefs.DefaultLanguage,
 			Status:         domain.StatusDownloading,
 			Title:          entry.Title,