feat(stats): consolidate per-user sync status into the Stats page

The Settings > Sync tab moves into the Stats module: Stats is now a per-user
page (Overview tab = sync status + your own API usage + manual actions) with the
admin instance-wide quota dashboard + background-sync pause as an admin-only
System tab. The Stats nav item is visible to all users (was admin-only); the
Settings Sync tab is removed. Sync i18n strings move from settings.sync.* to
stats.sync.* (EN/HU/DE).

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

@@ -3,7 +3,6 @@
   "tabs": {
     "appearance": "Appearance",
     "notifications": "Notifications",
-    "sync": "Sync",
     "account": "Account"
   },
   "save": {
@@ -42,43 +41,6 @@
     "testTitle": "Test notification",
     "testMessage": "This is what a notification looks like."
   },
-  "sync": {
-    "myStatus": "My sync status",
-    "channels": "Channels",
-    "channelsHint": "How many channels you're subscribed to.",
-    "recentSynced": "Recent synced",
-    "recentSyncedHint": "Channels whose recent uploads (~last year) are already in the local database, so they show in your feed.",
-    "fullHistory": "Full history",
-    "fullHistoryHint": "Channels whose entire back-catalog has been fetched, out of the ones you've requested full history for (opt in per channel on the Channels page).",
-    "fullHistoryEta": "Full history ETA",
-    "fullHistoryEtaHint": "Rough estimate to finish full-history backfill of {{count}} pending channel(s), based on the shared daily quota.",
-    "myVideos": "My videos",
-    "myVideosHint": "Total videos available to you across your subscribed channels.",
-    "quotaLeft": "Quota left today",
-    "quotaLeftHint": "Shared YouTube API budget remaining today (resets midnight US Pacific). Backfill yields to it.",
-    "loading": "Loading…",
-    "apiUsage": "Your API usage",
-    "today": "Today",
-    "todayHint": "YouTube API units your own actions used today (resets midnight US Pacific). Background sync isn't counted here.",
-    "last7d": "Last 7 days",
-    "last30d": "Last 30 days",
-    "allTime": "All time",
-    "byAction": "By action (30d)",
-    "noUsage": "No usage yet.",
-    "actions": "Actions",
-    "syncSubscriptions": "Sync subscriptions",
-    "syncSubscriptionsHint": "Re-import your YouTube subscriptions and pull in any newly-followed channels.",
-    "backfillEverything": "Backfill everything",
-    "backfillEverythingHint": "Request full back-catalog backfill for every channel you're subscribed to. Older videos + complete search fill in as the shared daily quota allows.",
-    "synced": "Synced {{count}} subscriptions",
-    "syncFailed": "Sync failed",
-    "fullHistoryRequested": "Full history requested for {{count}} channels",
-    "fullHistoryFailed": "Couldn't request full history",
-    "admin": "Admin",
-    "adminPauseHint": "Pause or resume the background sync for the whole app (all users).",
-    "resumeBackgroundSync": "Resume background sync",
-    "pauseBackgroundSync": "Pause background sync"
-  },
   "account": {
     "title": "Account",
     "youtubeAccess": "YouTube access",