feat(channels): data-table channel manager

Replace the stacked card rows with the reusable DataTable: sortable columns, in-header
Channel/Tags filters, status chips + page controls in one row, wider layout. Tag toggle
is now optimistic (no full refetch). Sync column collapses to one 'fully synced' chip;
Actions column carries hide/unsubscribe + a backfill control (admin reset / per-user
full-history opt-in). Channel status filter persists across reloads.

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

@@ -1,7 +1,7 @@
 {
   "intro": "Set a channel's <0>priority</0> to push its videos up when you sort by “Channel priority”, attach your own <1>tags</1> to filter the feed, or <2>hide</2> a channel to drop it from the feed without unsubscribing.",
   "filterPlaceholder": "Filter channels…",
-  "syncSubscriptions": "Sync subscriptions",
+  "syncSubscriptions": "Read subscriptions from YouTube",
   "syncSubscriptionsHint": "Re-import your subscription list from YouTube — adds channels you've newly followed and drops ones you've unfollowed. The videos themselves keep syncing automatically in the background; this does not re-fetch them.",
   "backfillEverything": "Backfill everything",
   "backfillEverythingHint": "Request full back-catalog backfill for every channel you're subscribed to. Older videos and search become complete as the shared daily quota allows — this can take a while.",
@@ -19,6 +19,20 @@
   },
   "loading": "Loading channels…",
   "empty": "No channels.",
+  "pager": {
+    "prev": "Previous",
+    "next": "Next",
+    "page": "Page {{page}} of {{total}}"
+  },
+  "cols": {
+    "priority": "Prio",
+    "channel": "Channel",
+    "stored": "Stored",
+    "subs": "Subscribers",
+    "sync": "Sync",
+    "tags": "Tags",
+    "actions": "Actions"
+  },
   "stats": {
     "channels": "Channels",
     "channelsHint": "Channels you're subscribed to.",
@@ -45,6 +59,13 @@
     "recentNotSyncedHint": "Recent uploads not fetched yet.",
     "full": "full",
     "fullHint": "Full history fetched.",
+    "fullNotFetchedHint": "Full back-catalog not fetched yet — use the backfill action in the Actions column.",
+    "fullySynced": "fully synced",
+    "fullySyncedHint": "Recent uploads and the full back-catalog are both in.",
+    "backfillThis": "Backfill this channel",
+    "backfillThisHint": "Fetch this channel's full back-catalog (older videos + complete search). Click again to cancel the request.",
+    "resetBackfill": "Reset & re-fetch this channel",
+    "resetHint": "Admin: re-fetch this channel from scratch (recent + full back-catalog), regardless of its current sync state.",
     "fullHistoryQueued": "full history queued",
     "fullHistoryComing": "full history coming",
     "queuedRequestedHint": "Full history requested — this channel's whole back-catalog will backfill as the shared quota allows. Click to cancel your request.",
@@ -67,7 +88,10 @@
     "fullHistoryRequested": "Full history requested for {{count}} channels",
     "fullHistoryFailed": "Couldn't request full history",
     "needYouTube": "Connect your YouTube account to do this.",
-    "connect": "Connect"
+    "connect": "Connect",
+    "resetDone": "Channel reset — re-fetching now",
+    "resetFailed": "Couldn't reset this channel"
   },
-  "confirmUnsubscribe": "Unsubscribe from \"{{name}}\" on YouTube? This changes your real YouTube account. To just remove it from your feed, hide it instead."
+  "confirmUnsubscribe": "Unsubscribe from \"{{name}}\" on YouTube? This changes your real YouTube account. To just remove it from your feed, hide it instead.",
+  "confirmReset": "Re-fetch \"{{name}}\" from scratch? This re-runs its backfill (recent + full history) and spends some API quota."
 }