fix: resolve back step, audio labels, progress parsing, file size pre-check, and stale job cleanup

- Fix handleBackStep to redirect to download prompt when step state is nil
  (previously silent no-op after container selection cleared the state)
- Improve formatLabel for audio-only entries without bitrate: use ID + extension
  instead of generic 'audio', fixing duplicate entries in format list
- Add yt-dlp --progress-template for reliable JSON progress parsing across all
  sites (fixes 0.0% progress on sites like xnxx.com where [download] lines
  are absent); keep regex parser as fallback for older yt-dlp versions
- Add file size pre-check in handleContainerSelection before download starts
  (previously checked only after download completed, wasting bandwidth)
- Clear stale active jobs and step state on /start for a clean slate

internal/handler/handler.go

@@ -442,9 +442,11 @@ func (h *Handler) generateJobID() string {
 	return uuid.NewString()[:8]
 }
 
-// handleStart shows the main menu.
+// handleStart shows the main menu and cleans up any stale state.
 func (h *Handler) handleStart(ctx context.Context, msg *models.Message, userID int64) {
 	slog.Info("start", "user_id", userID, "chat_id", msg.Chat.ID)
+	h.clearActiveJob(userID)
+	h.clearStepState(msg.Chat.ID)
 	h.sendWithKB(ctx, msg.Chat.ID, "Main Menu:", mainKeyboard())
 }