Scope check: pre-download playlist browser (pick items before queuing)

Given the scope line from #1027 ("MeTube writes the file once, as correctly as it can"), checking before sending a PR this time.

The idea: a Browse action next to Download/Subscribe for playlist/channel URLs. It flat-probes the URL (no download) and shows the entries as a list — title, duration, thumbnail — with checkboxes. The user picks items and "Add selected" queues each one as a normal individual download with the form's current settings. Nothing happens to files after they are written; this is purely about choosing what gets downloaded, for the case where you want 7 tracks out of a 400-item playlist and the item-limit slider can't express that.

The part I suspect you'll want to weigh: to badge entries as "already downloaded" (and offer a hide-them filter), my draft keeps a small persistent registry of finished source URLs (normalized, so youtu.be/music.youtube/shorts variants match) in STATE_DIR, updated when a download finishes. That's new persistent state, which I know you've been trimming. A slimmer variant drops the registry and only badges against the current completed list — weaker after a Clear, but zero new state.

Is either variant something you'd take, or is this on the wrong side of the line too? Happy to keep it in my fork if so — no hard feelings either way.