refactor: remove all playlist support, fix message ordering

- Remove playlist handler, keyboard, and all related callback routing
- Strip playlist parameter from YouTube Music radio URLs, reject pure playlists
- Send media preview before format selection prompt (correct ordering)
- Remove DetectPlaylist/GetPlaylistInfo methods (no longer needed)
- Remove IsPlaylist, PlaylistCount, PlaylistEntries from MediaInfo
- Remove PlaylistState struct, keep PlaylistEntry for search results
- Update README to reflect removal of playlist feature

internal/handler/handler.go

@@ -281,14 +281,6 @@ func (h *Handler) routePrefixedCallback(ctx context.Context, chatID int64, msgID
 		h.handleLanguageSelection(ctx, chatID, msgID, userID, data[5:])
 	case strings.HasPrefix(data, "container_"):
 		// container selection was removed — yt-dlp handles it
-	case strings.HasPrefix(data, "pl_page_"):
-		h.handlePlaylistPage(ctx, chatID, msgID, userID, data[8:])
-	case strings.HasPrefix(data, "pl_entry_"):
-		h.handlePlaylistEntry(ctx, chatID, msgID, userID, data[9:])
-	case data == "pl_select_all":
-		h.handlePlaylistSelectAll(ctx, chatID, msgID, userID)
-	case data == "pl_confirm":
-		h.handlePlaylistConfirm(ctx, chatID, msgID, userID)
 	case strings.HasPrefix(data, "delaccount_"):
 		if uid, err := strconv.ParseInt(data[11:], 10, 64); err == nil && uid > 0 {
 			h.deleteAccountConfirm(ctx, chatID, msgID, uid)