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/main.tsx

@@ -2,6 +2,7 @@ import React from "react";
 import { createRoot } from "react-dom/client";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import App from "./App";
+import ErrorBoundary from "./components/ErrorBoundary";
 import "./index.css";
 
 const queryClient = new QueryClient({
@@ -11,7 +12,9 @@ const queryClient = new QueryClient({
 createRoot(document.getElementById("root")!).render(
   <React.StrictMode>
     <QueryClientProvider client={queryClient}>
-      <App />
+      <ErrorBoundary>
+        <App />
+      </ErrorBoundary>
     </QueryClientProvider>
   </React.StrictMode>
 );