Named playlists and compilations - #999

The existing "playlist" was a single, unnamed list that could only be built
by filtering the gallery. This adds any number of named playlists, managed
from a sidebar panel (Settings > "Show playlists"):

- Create, rename, duplicate, reorder (drag) and delete playlists.
- Add videos from the right-click menu ("Add to playlist..."), from the
  thumbnail's playlist icon (adds to the last used playlist), or by dragging
  thumbnails onto a playlist; "Add N selected" adds a whole selection.
- Open a playlist to see its entries in order: drag to reorder, Delete to
  remove, Ctrl+A to select, move or copy entries to another playlist, open
  the containing folder. "Play" / "Shuffle play" hand the list to the
  preferred external player as an .m3u8.
- A "compilation" is a playlist whose entries can carry a start/end time
  (a highlight). This commit stores and plays those ranges externally
  (#EXTVLCOPT start/stop times); marking them comes with the built-in player.

Storage: playlists live in a sidecar file next to the hub
(`<hub>.playlists.json`, versioned, written atomically with a .bak), so the
.vha2 format is untouched and a hub without the sidecar behaves exactly as
before. Entries reference videos by hash and re-link by path when a file
was moved and rescanned. The legacy "playlist" timestamp field is migrated
into a playlist named "Playlist" the first time the hub is opened.

An open playlist overlays the gallery; "All videos", the gallery button and
Esc always lead back, so a new or empty playlist can never trap the view.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>