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.

frontend/src/components/PlayerModal.dom.test.tsx

@@ -216,6 +216,11 @@ describe("PlayerModal — removing the playing video from its playlist", () => {
     expect(lastStart()?.videoId).toBe("C");
     prev();
     await waitFor(() => expect(lastStart()?.videoId).toBe("B"));
+    // …and A is gone from the queue, not just skipped over: B is its head now, so stepping back
+    // again goes nowhere. (One step back lands on B either way — it took the second to tell.)
+    prev();
+    await settle();
+    expect(lastStart()?.videoId).toBe("B");
     expect(started.filter((s) => s.videoId === "A")).toHaveLength(1); // never played again
   });