feat(notifications): link the subscribe activity to the channel

The "Subscribed on YouTube" inbox entry now names the channel and offers
two convenience links that survive a reload (driven by the typed payload,
not the live callback): "Channel manager" (jumps to the manager focused on
that channel) and "Open on YouTube". Wires the panel to the app's
focus-channel navigation; trilingual strings.

frontend/src/App.tsx

@@ -451,7 +451,7 @@ export default function App() {
           ) : page === "playlists" ? (
             <Playlists canWrite={meQuery.data!.can_write} />
           ) : page === "notifications" ? (
-            <NotificationsPanel filters={filters} setFilters={setFilters} setPage={setPage} />
+            <NotificationsPanel filters={filters} setFilters={setFilters} setPage={setPage} onFocusChannel={focusChannel} />
           ) : page === "settings" ? (
             <SettingsPanel
               me={meQuery.data!}