feat(ui): notification center with leveled toasts and error capture

- Replace the toast store with a notification store: levels (info/success/
  warning/error/fatal), requiresInteraction, and a persisted history.
- Move toasts to the top-right, styled per level, with manual dismiss.
- Add a bell in the header opening a Notification Center (history, unread badge,
  'needs attention' vs info, clear all).
- Capture network failures and 5xx responses (api layer) and render crashes
  (ErrorBoundary) as notifications.
- Sound + server-sourced events + per-account settings remain for 6b.

frontend/src/components/Header.tsx

@@ -12,6 +12,7 @@ import {
 import { SCHEMES, type Scheme, type ThemePrefs } from "../lib/theme";
 import type { FeedFilters, Me } from "../lib/api";
 import SyncStatus from "./SyncStatus";
+import NotificationCenter from "./NotificationCenter";
 
 function IconBtn(props: React.ButtonHTMLAttributes<HTMLButtonElement>) {
   const { className = "", ...rest } = props;
@@ -130,6 +131,8 @@ export default function Header({
           )}
         </div>
 
+        <NotificationCenter />
+
         <div className="pl-1">
           <AccountMenu me={me} logout={logout} />
         </div>