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/components/Feed.tsx

@@ -174,7 +174,13 @@ export default function Feed({
                 ? i18n.t("feed.markedWatchedNamed", { title: v.title })
                 : i18n.t("feed.markedWatched"),
               action: { label: i18n.t("feed.unwatch"), onClick: () => onState(id, "new") },
-              meta: { kind: "video-watched", videoId: id, title: v?.title ?? "this video" },
+              meta: {
+                kind: "video-watched",
+                videoId: id,
+                title: v?.title ?? "this video",
+                channelId: v?.channel_id ?? "",
+                channelName: v?.channel_title ?? "",
+              },
             });
           } else if (status === "new") {
             // Unhide / unwatch (from a card, the toast's Undo, or the center): quietly resolve