fix(playlists): the player's Remove follows the list it was launched from, the player survives a change of selection, and a gone list is said in the reader's language

- D3 snapshotted the removal's TARGET, but whether the player offered Remove still followed the
  selected list: launched from Watch later (no Remove, by design), selecting another list under it
  made Remove appear — and delete from Watch later. The snapshot now carries the launch list's
  editability too.
- Selecting another list under an open player (the command palette works over it) put the page into
  "Loading…", and the player was rendered only in the loaded branch — it vanished mid-playback and
  came back at its launch video. It now sits at one place in the tree whatever the page is doing.
- A removal takes the video out of the CACHED list as well as the rows: the player's progress
  checkpoint rewrote that entry before the refetch landed, handing the membership guard a list that
  still held the video — the row came back and the undo history went.
- The watched-state callback refreshed the selected list, not the launch list (D3's sibling).
- `_own_playlist`'s 404 is structured (`{"code": "playlist", "reason": "not_found"}`) and localised:
  the bare "Unknown playlist" reached the Hungarian UI in English, which also made round 1's new
  `removeFailed` string unreachable. Both Remove and Move report through `notifyActionError` now.
- The "viewer moved on under the confirm" test stepped back only once, which lands on the same video
  whether or not the removed one left the queue; it now takes the second step that tells them apart.

backend/app/routes/playlists.py

@@ -50,7 +50,10 @@ def _check_pull_cooldown(user: User) -> None:
 def _own_playlist(db: Session, user: User, playlist_id: int) -> Playlist:
     pl = db.get(Playlist, playlist_id)
     if pl is None or pl.user_id != user.id:
-        raise HTTPException(status_code=404, detail="Unknown playlist")
+        # Structured, so the UI can say it in the reader's language: this 404 is exactly the one a
+        # user meets in practice (the list was deleted in another tab), and a bare "Unknown
+        # playlist" reached the Hungarian UI in English. One reason for "gone" and "not yours" alike.
+        raise HTTPException(status_code=404, detail={"code": "playlist", "reason": "not_found"})
     return pl