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/header.json

@@ -40,12 +40,15 @@
     "total": "total",
     "countTooltip": "Your subscriptions vs. the whole shared catalog.",
     "paused": "paused",
-    "syncing": "{{count}} syncing",
+    "syncing_one": "{{count}} syncing",
+    "syncing_other": "{{count}} syncing",
     "backfillingHistory": "fetching history",
     "working": "syncing…",
-    "recentQueued": "{{count}} queued",
+    "recentQueued_one": "{{count}} queued",
+    "recentQueued_other": "{{count}} queued",
     "allSynced": "all synced",
-    "withoutFullHistory": "{{count}} without full history",
+    "withoutFullHistory_one": "{{count}} without full history",
+    "withoutFullHistory_other": "{{count}} without full history",
     "fullHistoryTooltip": "Recent uploads only — click to fetch the rest.",
     "pause": "Pause sync",
     "resume": "Resume sync"