refactor(channels): share one ChannelLink + channelYouTubeUrl helper

The "avatar + name + open-on-YouTube" cell and the @handle-or-/channel/<id>
URL were copy-pasted across the channel manager, the discovery tab, the
subscribe notice and the player. Extract a single ChannelLink component
(optional in-app onView, middle-click opens YouTube) and a channelYouTubeUrl
helper, and route all four through them. Removes the NameCell / DiscoveryNameCell
duplication (the latter introduced with the discovery tab).

frontend/src/components/ChannelLink.tsx

@@ -0,0 +1,64 @@
+import { useTranslation } from "react-i18next";
+import { ExternalLink } from "lucide-react";
+import Avatar from "./Avatar";
+import Tooltip from "./Tooltip";
+import { channelYouTubeUrl } from "../lib/format";
+
+// Avatar + channel name + "open on YouTube" link, shared by the Channel manager and the
+// discovery list (and anywhere else that lists a channel). When `onView` is given the name
+// is a button that opens the in-app channel view, with middle-click opening YouTube in a new
+// tab; without it the name is plain text.
+export default function ChannelLink({
+  id,
+  title,
+  handle,
+  thumbnailUrl,
+  onView,
+}: {
+  id: string;
+  title: string | null;
+  handle?: string | null;
+  thumbnailUrl: string | null;
+  onView?: () => void;
+}) {
+  const { t } = useTranslation();
+  const ytUrl = channelYouTubeUrl(id, handle);
+  const name = title ?? id;
+  return (
+    <div className="flex items-center gap-2 min-w-0">
+      <Avatar src={thumbnailUrl} fallback={title ?? ""} className="w-8 h-8 rounded-full shrink-0" />
+      {onView ? (
+        <button
+          onClick={onView}
+          onAuxClick={(e) => {
+            // Middle-click opens the channel's YouTube page in a new tab; left-click keeps
+            // opening the in-app channel detail.
+            if (e.button === 1) {
+              e.preventDefault();
+              window.open(ytUrl, "_blank", "noopener,noreferrer");
+            }
+          }}
+          onMouseDown={(e) => {
+            if (e.button === 1) e.preventDefault(); // suppress middle-click autoscroll
+          }}
+          className="text-sm font-medium truncate hover:text-accent text-left min-w-0"
+        >
+          {name}
+        </button>
+      ) : (
+        <span className="text-sm font-medium truncate min-w-0">{name}</span>
+      )}
+      <Tooltip hint={t("channels.row.openOnYouTube")}>
+        <a
+          href={ytUrl}
+          target="_blank"
+          rel="noopener noreferrer"
+          className="text-muted hover:text-accent shrink-0"
+          aria-label={t("channels.row.openOnYouTube")}
+        >
+          <ExternalLink className="w-3.5 h-3.5" />
+        </a>
+      </Tooltip>
+    </div>
+  );
+}