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/i18n/locales/de/inbox.json

@@ -10,6 +10,7 @@
   "sectionSystem": "System",
   "sectionActivity": "Aktivität",
   "andMore": "und {{count}} weitere",
+  "openScheduler": "Zeitplaner öffnen",
   "maintenance": {
     "title": "Videos entfernt",
     "body_one": "{{count}} gespeichertes oder in einer Playlist befindliches Video wurde entfernt, weil es auf YouTube nicht mehr verfügbar ist.",

frontend/src/i18n/locales/en/inbox.json

@@ -10,6 +10,7 @@
   "sectionSystem": "System",
   "sectionActivity": "Activity",
   "andMore": "and {{count}} more",
+  "openScheduler": "Open Scheduler",
   "maintenance": {
     "title": "Videos removed",
     "body_one": "{{count}} saved or playlisted video was removed because it's no longer available on YouTube.",

frontend/src/i18n/locales/hu/inbox.json

@@ -10,6 +10,7 @@
   "sectionSystem": "Rendszer",
   "sectionActivity": "Tevékenység",
   "andMore": "és még {{count}}",
+  "openScheduler": "Ütemező megnyitása",
   "maintenance": {
     "title": "Videók eltávolítva",
     "body_one": "{{count}} mentett vagy lejátszási listás videó eltávolítva, mert már nem elérhető a YouTube-on.",