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/i18n/locales/en/playlists.json

@@ -77,6 +77,9 @@
   "movedToast": "“{{title}}” moved to “{{name}}” ✓",
   "moveFailed": "Couldn't move the video.",
   "removeFailed": "Couldn't remove the video.",
+  "errors": {
+    "not_found": "This playlist doesn't exist any more."
+  },
   "moveErrors": {
     "not_in_playlist": "That video isn't in this playlist any more — it changed somewhere else.",
     "same_playlist": "That video is already in this playlist."

frontend/src/i18n/locales/hu/playlists.json

@@ -77,6 +77,9 @@
   "movedToast": "„{{title}}” áthelyezve ide: „{{name}}” ✓",
   "moveFailed": "Nem sikerült áthelyezni a videót.",
   "removeFailed": "Nem sikerült eltávolítani a videót.",
+  "errors": {
+    "not_found": "Ez a lejátszási lista már nem létezik."
+  },
   "moveErrors": {
     "not_in_playlist": "Ez a videó már nincs ebben a listában — időközben máshol megváltozott.",
     "same_playlist": "Ez a videó már ebben a listában van."