feat(inbox): add navigation links to watched and scheduler notices

Extend the reload-safe meta+link pattern (introduced for "Subscribed") to
more inbox entries, so every notice that can point somewhere does:

- A "Marked watched" activity notice now offers "Find in feed" (jumping to
  the feed filtered to Watched + that channel), alongside Unwatch — matching
  what the "Hidden" notice already does. VideoWatchedMeta carries the
  channel, and locate() handles both hidden and watched.
- A scheduler job-completion notice (System) now offers "Open Scheduler",
  jumping to the dashboard.

Maintenance notices reference already-removed videos, so they stay
informational with nothing to navigate to.

frontend/src/lib/notifications.ts

@@ -23,6 +23,8 @@ export type VideoWatchedMeta = {
   kind: "video-watched";
   videoId: string;
   title: string;
+  channelId: string;
+  channelName: string;
 };
 export type ChannelSubscribedMeta = {
   kind: "channel-subscribed";