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/tests/test_playlist_move.py

@@ -128,6 +128,22 @@ def test_move_into_someone_elses_list_404s_and_changes_nothing(db, make_user, as
     assert _ids(db, theirs.id) == []
 
 
+def test_a_list_that_is_not_yours_answers_with_a_reason_the_ui_can_translate(
+    db, make_user, as_user
+):
+    # The one 404 a user meets in practice (the list went away in another tab). It used to be a bare
+    # English "Unknown playlist", which the Hungarian UI then showed verbatim.
+    user, other = make_user(), make_user()
+    _seed_videos(db, ["v1"])
+    theirs = _playlist(db, other, "Theirs", ["v1"])
+
+    r = as_user(user).delete(f"/api/playlists/{theirs.id}/items/v1")
+    assert r.status_code == 404
+    assert r.json()["detail"] == {"code": "playlist", "reason": "not_found"}
+    db.expire_all()
+    assert _ids(db, theirs.id) == ["v1"]
+
+
 def test_move_marks_both_linked_sides_dirty(db, make_user, as_user):
     """A move edits two YouTube mirrors at once — the source lost an item, the target gained one.
     Marking only the source is the asymmetric fix this repo keeps re-learning."""