test(i18n): every locale key must be read by the app; 75 dead keys removed

A contract test scans the source (tests excluded) for each key as a whole literal, or under a
dynamic prefix like `plex.sort.${k}`, with plural forms counted through their base key. It found 75
keys that had outlived the code showing them (the old sidebar sort vocabulary, the old login page, a
pager the DataTable replaced, …); they are gone from EN and HU. A key read in a way the scan cannot
see goes in an explicit ALLOWED map with its reason.

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

@@ -1,6 +1,5 @@
 {
   "navLabel": "Plex",
-  "backToFeed": "YouTube Feed",
   "backToLibrary": "Back to library",
   "collection": {
     "playAll": "Play all",
@@ -113,8 +112,6 @@
       "transcode_sw": "Transcode · SW"
     }
   },
-  "count": "{{count}} titles",
-  "searchCount": "{{count}} matches",
   "noMatches": "No matches.",
   "noMatchesFiltered_one": "No matches — a filter is also narrowing the results.",
   "noMatchesFiltered_other": "No matches — {{count}} filters are also narrowing the results.",
@@ -303,10 +300,8 @@
   },
   "playlist": {
     "section": "Playlists",
-    "newPlaceholder": "New playlist…",
     "pickerPlaceholder": "Find or create a playlist…",
     "createNamed": "Create \"{{name}}\"",
-    "create": "Create playlist",
     "none": "No playlists yet.",
     "deleteConfirm": "Delete the playlist \"{{title}}\"?",
     "rename": "Rename",