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/ChannelDiscovery.tsx

@@ -1,11 +1,11 @@
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
-import { ExternalLink, UserPlus } from "lucide-react";
+import { UserPlus } from "lucide-react";
 import { api, HttpError, type DiscoveredChannel } from "../lib/api";
 import { formatViews } from "../lib/format";
 import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
-import Avatar from "./Avatar";
+import ChannelLink from "./ChannelLink";
 import DataTable, { type Column } from "./DataTable";
 
 // The Channel manager's "Discovery" tab: channels that turn up in the user's playlists but
@@ -70,7 +70,9 @@ export default function ChannelDiscovery({
       sortValue: (c) => (c.title ?? c.id).toLowerCase(),
       filter: { kind: "text", get: (c) => `${c.title ?? ""} ${c.handle ?? ""}` },
       cardPrimary: true,
-      render: (c) => <DiscoveryNameCell c={c} />,
+      render: (c) => (
+        <ChannelLink id={c.id} title={c.title} handle={c.handle} thumbnailUrl={c.thumbnail_url} />
+      ),
     },
     {
       key: "subs",
@@ -153,27 +155,3 @@ export default function ChannelDiscovery({
     </div>
   );
 }
-
-function DiscoveryNameCell({ c }: { c: DiscoveredChannel }) {
-  const { t } = useTranslation();
-  const ytUrl = c.handle
-    ? `https://www.youtube.com/@${c.handle.replace(/^@/, "")}`
-    : `https://www.youtube.com/channel/${c.id}`;
-  return (
-    <div className="flex items-center gap-2 min-w-0">
-      <Avatar src={c.thumbnail_url} fallback={c.title ?? ""} className="w-8 h-8 rounded-full shrink-0" />
-      <span className="text-sm font-medium truncate min-w-0">{c.title ?? c.id}</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>
-  );
-}