fix(i18n): every counted string is a plural pair, so one reads singular

"1 seasons", "Synced 1 subscriptions", "1 video(s) were skipped": 21 English
strings carried {{count}} in one plain key and read plural at one. Each is a
_one/_other pair now (Hungarian keeps one form for both), and so is every other
counted string, so the next wording change is a plural too. A contract test
holds the rule for every locale file.

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

@@ -128,7 +128,8 @@
   "seasonTitle": "Season {{n}}",
   "specials": "Specials",
   "watchedFailed": "Couldn't update the watched state — please try again",
-  "seasons": "{{count}} seasons",
+  "seasons_one": "{{count}} season",
+  "seasons_other": "{{count}} seasons",
   "unified": {
     "titles": "Titles",
     "episodes": "Episodes"
@@ -136,7 +137,8 @@
   "series": {
     "seasons": "Seasons",
     "backToShow": "Back to show",
-    "episodeCount": "{{count}} episodes",
+    "episodeCount_one": "{{count}} episode",
+    "episodeCount_other": "{{count}} episodes",
     "showFromStart": "Show from the start",
     "seasonFromStart": "Season from the start",
     "related": "Related shows",
@@ -154,11 +156,13 @@
     "create": "Create",
     "search": "Filter collections…",
     "none": "No editable collections yet. Create one above.",
-    "count": "{{count}} items",
+    "count_one": "{{count}} item",
+    "count_other": "{{count}} items",
     "delete": "Delete",
     "deleteConfirm": "Delete the collection “{{title}}” from Plex? This affects all users.",
     "note": "Changes are written to Plex and shared across all clients and users.",
-    "others": "Manage {{count}} existing collections",
+    "others_one": "Manage {{count}} existing collection",
+    "others_other": "Manage {{count}} existing collections",
     "makeEditable": "Manage"
   },
   "player": {
@@ -298,7 +302,8 @@
     "newPlaceholder": "New playlist name…",
     "create": "Create",
     "none": "No playlists yet. Create one above.",
-    "count": "{{count}} items",
+    "count_one": "{{count}} item",
+    "count_other": "{{count}} items",
     "groupProgress": "{{in}} / {{size}}",
     "addFailed": "Couldn't add it to the playlist",
     "removeFailed": "Couldn't remove it from the playlist",
@@ -319,7 +324,8 @@
     "layoutTree": "Tree view",
     "removeShow": "Remove whole show",
     "removeSeason": "Remove whole season",
-    "episodes": "{{count}} episodes",
+    "episodes_one": "{{count}} episode",
+    "episodes_other": "{{count}} episodes",
     "seasonUnknown": "Episodes",
     "addShow": "Add whole show to a playlist",
     "addSeason": "Add season to a playlist",
@@ -345,8 +351,10 @@
   "person": {
     "credits": "{{movies}} movies · {{shows}} shows",
     "filterBy": "Filter library by {{name}}",
-    "movies": "Movies ({{count}})",
-    "shows": "Shows ({{count}})",
+    "movies_one": "Movies ({{count}})",
+    "movies_other": "Movies ({{count}})",
+    "shows_one": "Shows ({{count}})",
+    "shows_other": "Shows ({{count}})",
     "empty": "No titles found for this person.",
     "born": "Born",
     "died": "Died",