chore(downloads-ui): FE-C4 follow-up cleanup

- Delete dead top-level downloads.rename.* i18n block (en/hu/de); the
  metadata modal moved to downloads.edit.*. Keep downloads.actions.rename
  (still used by ProfileEditor).
- Rename misnamed state renameJob/setRenameJob -> metaJob/setMetaJob in
  DownloadCenter (it opens MetaEditModal, not a rename dialog).
- Extract the duplicated [downloads]/[download-index]/[download-usage]
  invalidation trio into lib/downloads.ts invalidateDownloads(qc);
  used by DownloadCenter and DownloadDialog.

Behavior-neutral. tsc green, knip shows no new unused export, JSON valid.

frontend/src/i18n/locales/de/downloads.json

@@ -78,12 +78,6 @@
     "copied": "Quell-Link in die Zwischenablage kopiert",
     "copyFailed": "Link konnte nicht kopiert werden"
   },
-  "rename": {
-    "title": "Download umbenennen",
-    "label": "Anzeigename",
-    "hint": "Ändert nur den angezeigten und beim Speichern verwendeten Namen — die gespeicherte Datei behält ihren Namen.",
-    "save": "Speichern"
-  },
   "edit": {
     "title": "Details bearbeiten",
     "name": "Titel",