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/App.tsx

@@ -252,8 +252,6 @@ export default function App() {
         onOpenAbout={() => setAboutOpen(true)}
         onChangeLanguage={changeLanguage}
         language={i18n.language as LangCode}
-        filters={filters}
-        setFilters={setFilters}
       />
       <div className="relative flex-1 min-w-0 flex flex-col">
         <Header
@@ -305,7 +303,7 @@ export default function App() {
           ) : page === "playlists" ? (
             <Playlists canWrite={meQuery.data!.can_write} />
           ) : page === "notifications" ? (
-            <NotificationsPanel />
+            <NotificationsPanel filters={filters} setFilters={setFilters} setPage={setPage} />
           ) : page === "settings" ? (
             <SettingsPanel
               me={meQuery.data!}