fix(ui): notification UX — outside-click close, toast countdown, hide entry actions

- NotificationCenter closes on outside click (document listener, not an overlay
  that the blurred header trapped) and no longer needs a second bell click.
- Toasts show a level-colored countdown bar and auto-dismiss faster (6s default).
- Hidden-video notifications carry structured meta so the center offers an in-app
  'Find in feed' (jump to that channel's hidden videos) and a one-click 'Unhide',
  working even after a reload when the live Undo callback is gone.

frontend/src/components/Header.tsx

@@ -131,7 +131,7 @@ export default function Header({
           )}
         </div>
 
-        <NotificationCenter />
+        <NotificationCenter filters={filters} setFilters={setFilters} />
 
         <div className="pl-1">
           <AccountMenu me={me} logout={logout} />