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/channels.json

@@ -94,7 +94,8 @@
     "fullHistory": "Full history",
     "fullHistoryHint": "Channels whose entire back-catalog is fetched, out of the ones you've requested full history for.",
     "left": "left",
-    "leftHint": "Rough estimate to finish full-history backfill of {{count}} pending channel(s), based on the shared daily quota.",
+    "leftHint_one": "Rough estimate to finish full-history backfill of {{count}} pending channel, based on the shared daily quota.",
+    "leftHint_other": "Rough estimate to finish full-history backfill of {{count}} pending channels, based on the shared daily quota.",
     "myVideos": "My videos",
     "myVideosHint": "Total videos available across your channels.",
     "quotaLeft": "Quota left",
@@ -125,11 +126,13 @@
     "noTagsYet": "No tags yet — make one with a row’s tag “+”"
   },
   "notify": {
-    "synced": "Synced {{count}} subscriptions",
+    "synced_one": "Synced {{count}} subscription",
+    "synced_other": "Synced {{count}} subscriptions",
     "syncFailed": "Couldn't sync subscriptions",
     "unsubscribed": "Unsubscribed from {{name}} on YouTube",
     "unsubscribeFailed": "Couldn't unsubscribe",
-    "fullHistoryRequested": "Full history requested for {{count}} channels",
+    "fullHistoryRequested_one": "Full history requested for {{count}} channel",
+    "fullHistoryRequested_other": "Full history requested for {{count}} channels",
     "fullHistoryFailed": "Couldn't request full history",
     "needYouTube": "Connect your YouTube account to do this",
     "connect": "Connect",