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/NotificationsPanel.tsx

@@ -18,6 +18,7 @@ import {
   type VideoHiddenMeta,
   type VideoWatchedMeta,
 } from "../lib/notifications";
+import { channelYouTubeUrl } from "../lib/format";
 import type { Page } from "../lib/urlState";
 import { LEVEL_STYLE } from "./Toaster";
 
@@ -380,7 +381,7 @@ function ClientActivityRow({
               {t("notifications.openInManager")}
             </button>
             <a
-              href={`https://www.youtube.com/channel/${subscribed.channelId}`}
+              href={channelYouTubeUrl(subscribed.channelId)}
               target="_blank"
               rel="noopener noreferrer"
               className="flex items-center gap-1 text-accent text-sm font-semibold hover:underline"