feat(notifications): unify the two rail indicators into one inbox

Fold the client-side transient bell into the inbox page so there's a single
notification indicator. The inbox now has two groups — "System" (durable,
server-backed) and "Activity" (client-side events with their Unhide/Unwatch/
Find-in-feed actions) — and the nav badge sums both unread counts. The separate
rail bell (NotificationCenter) is removed.

Activity items get a per-item clear (X) alongside the global Clear all. Unhiding
or unwatching a video from anywhere — a card, the toast's Undo, or the inbox —
now quietly resolves the original "Hidden/Watched X" entry (no duplicate "Unhidden
X" toast, no stale entry with a dead action), via a new resolveVideo store helper.

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

@@ -7,6 +7,8 @@
   "clearAll": "Alle löschen",
   "markRead": "Als gelesen markieren",
   "dismiss": "Verwerfen",
+  "sectionSystem": "System",
+  "sectionActivity": "Aktivität",
   "andMore": "und {{count}} weitere",
   "maintenance": {
     "title": "Videos entfernt",