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/i18n/locales/de/notifications.json

@@ -8,6 +8,8 @@
   "findInFeed": "Im Feed finden",
   "unhide": "Einblenden",
   "unwatch": "Nicht angesehen",
+  "openInManager": "Kanalverwaltung",
+  "openOnYouTube": "Auf YouTube öffnen",
   "unhidden": "Eingeblendet „{{title}}”",
   "unwatched": "Als nicht angesehen markiert „{{title}}”",
   "time": {