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/playlists.json

@@ -4,7 +4,8 @@
   "listGroup": "Your playlists",
   "watchLater": "Watch later",
   "syncYoutube": "Sync from YouTube",
-  "syncedToast": "Synced {{count}} playlists from YouTube",
+  "syncedToast_one": "Synced {{count}} playlist from YouTube",
+  "syncedToast_other": "Synced {{count}} playlists from YouTube",
   "syncFailed": "Couldn't sync from YouTube",
   "noneYet": "No playlists yet",
   "newPlaylist": "New playlist…",
@@ -34,7 +35,8 @@
   "revertFailed": "Couldn't sync from YouTube",
   "pushTitle": "Sync to YouTube",
   "pushConfirm": "Sync",
-  "pushPlanCreate": "Create a new YouTube playlist with {{count}} videos? (~{{units}} quota units; {{left}} left today.)",
+  "pushPlanCreate_one": "Create a new YouTube playlist with {{count}} video? (~{{units}} quota units; {{left}} left today.)",
+  "pushPlanCreate_other": "Create a new YouTube playlist with {{count}} videos? (~{{units}} quota units; {{left}} left today.)",
   "mergeAddLocal_one": "{{count}} video added to Siftlode from YouTube.",
   "mergeAddLocal_other": "{{count}} videos added to Siftlode from YouTube.",
   "mergeRemoveLocal_one": "{{count}} video removed from Siftlode.",
@@ -53,7 +55,8 @@
   "pushNoQuota": "Not enough YouTube quota left today ({{left}}) for this sync (~{{units}} needed). Try again tomorrow.",
   "pushUpToDate": "“{{name}}” is already in sync with YouTube",
   "pushDone": "“{{name}}” synced to YouTube",
-  "pushPartial": "Synced, but {{count}} video(s) were skipped",
+  "pushPartial_one": "Synced, but {{count}} video was skipped",
+  "pushPartial_other": "Synced, but {{count}} videos were skipped",
   "pushFailed": "Couldn't sync to YouTube",
   "deleteHereOnly": "Delete here only",
   "deleteYtFailed": "Couldn't delete on YouTube; deleted here only",
@@ -103,7 +106,8 @@
   "dragBlockedFilter": "Reordering by hand is off while a filter is applied.",
   "clearView": "Reset view",
   "saveOrder": "Save this order",
-  "filteredCount": "{{count}} shown",
+  "filteredCount_one": "{{count}} shown",
+  "filteredCount_other": "{{count}} shown",
   "noneMatchFilter": "No videos match the filter.",
   "loadFailed": "Couldn't load this playlist",
   "reorderFailed": "Couldn't save the new order",