fix(playlists): "+N without length" counts every video the sum left out

The label said "without length" but the count held only live videos, so
a video not fetched yet, or gone before it was, was left out of the sum
and named nowhere. The server's `Video.without_length` and the client's
`isWithoutLength` are now one rule (no length, whatever the live
status), and the field, the key and the helpers are named for what they
count. The comments that still quoted "+N live" say the current copy.

frontend/src/lib/api/playlists.ts

@@ -17,8 +17,8 @@ export interface Playlist {
   dirty: boolean; // linked local playlist has edits not yet pushed to YouTube
   item_count: number;
   total_duration_seconds: number;
-  /** Live videos the total cannot hold (no length): the "+N without length" beside it. */
-  live_without_length: number;
+  /** The videos the total left out for having no length: the "+N without length" beside it. */
+  without_length: number;
   cover_thumbnail: string | null;
   updated_at: string; // ISO; when the list last CHANGED (rename, add/remove/move/reorder)
   has_video?: boolean; // only set when listed with ?contains=<video_id>