refactor: address /code-review on R11 feedback symmetry

- admin._decide: add an idempotency guard (409 unless status was 'pending') so a
  repeat/stale approve OR deny can't re-run its side effects — now that deny emails,
  this stops duplicate denial mails and a rejection mail to an already-approved user.
- SavedViewsWidget: extract runViewMutation(fn, failMsg) (returns success) — dedups
  the five try/catch+refresh blocks AND lets create/rename close their input only on
  success, so a failed create keeps the user's typed name.
- worker._notify_download_done: take user_id/title captured BEFORE the caller's commit
  (no post-commit refresh SELECT); add a  so the inbox labels an edit job
  'Edit ready' instead of 'Download ready' (new inbox.editDone i18n, HU+EN).

Gate green (ruff/tsc/eslint-0/prettier/knip/vitest 79/pytest 193); smoke-tested the
saved-views + notifications surfaces render clean on a fresh dev server.

frontend/src/i18n/locales/en/inbox.json

@@ -17,6 +17,9 @@
   "downloadDone": {
     "title": "Download ready"
   },
+  "editDone": {
+    "title": "Edit ready"
+  },
   "maintenance": {
     "title": "Videos removed",
     "body_one": "{{count}} saved or playlisted video was removed because it's no longer available on YouTube.",

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

@@ -17,6 +17,9 @@
   "downloadDone": {
     "title": "Letöltés kész"
   },
+  "editDone": {
+    "title": "Vágás kész"
+  },
   "maintenance": {
     "title": "Videók eltávolítva",
     "body_one": "{{count}} mentett vagy lejátszási listás videó eltávolítva, mert már nem elérhető a YouTube-on.",