feat(discovery): total-videos column + subscribe confirmation

Add a "Videos" column (channel total uploads, already in the discovery
response) to the Discover-from-playlists table, and guard the Subscribe button
with a confirm dialog warning it changes the real YouTube account and spends
quota (mirrors the unsubscribe guard). Strings added in EN/HU/DE.

frontend/src/components/ChannelDiscovery.tsx

@@ -7,6 +7,7 @@ import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import ChannelLink from "./ChannelLink";
 import DataTable, { type Column } from "./DataTable";
+import { useConfirm } from "./ConfirmProvider";
 
 // The Channel manager's "Discovery" tab: channels that turn up in the user's playlists but
 // that they don't subscribe to. Subscribing here only creates the subscription + enriches
@@ -20,6 +21,7 @@ export default function ChannelDiscovery({
 }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
+  const confirm = useConfirm();
 
   const query = useQuery({
     queryKey: ["discovered-channels"],
@@ -60,6 +62,17 @@ export default function ChannelDiscovery({
     },
   });
 
+  // Subscribing changes the user's real YouTube account and spends a little quota — confirm
+  // first (mirrors the unsubscribe guard in the Subscriptions tab).
+  const onSubscribe = async (c: DiscoveredChannel) => {
+    const ok = await confirm({
+      title: t("channels.discovery.subscribe"),
+      message: t("channels.discovery.confirmSubscribe", { name: c.title ?? c.id }),
+      confirmLabel: t("channels.discovery.subscribe"),
+    });
+    if (ok) subscribe.mutate(c);
+  };
+
   const rows = query.data ?? [];
 
   const columns: Column<DiscoveredChannel>[] = [
@@ -87,6 +100,19 @@ export default function ChannelDiscovery({
         </span>
       ),
     },
+    {
+      key: "videos",
+      header: t("channels.discovery.cols.totalVideos"),
+      align: "right",
+      nowrap: true,
+      sortable: true,
+      sortValue: (c) => c.video_count ?? -1,
+      render: (c) => (
+        <span className="text-muted tabular-nums">
+          {c.video_count != null ? formatViews(c.video_count) : "—"}
+        </span>
+      ),
+    },
     {
       key: "inPlaylists",
       header: t("channels.discovery.cols.inPlaylists"),
@@ -123,7 +149,7 @@ export default function ChannelDiscovery({
           }
         >
           <button
-            onClick={() => subscribe.mutate(c)}
+            onClick={() => onSubscribe(c)}
             disabled={!canWrite || subscribe.isPending}
             className="glass-card glass-hover inline-flex items-center gap-1.5 px-2.5 py-1.5 rounded-lg text-xs disabled:opacity-50 transition"
           >