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/lib/api.ts

@@ -1,3 +1,5 @@
+import { notify } from "./notifications";
+
 export interface Me {
   id: number;
   email: string;
@@ -67,12 +69,34 @@ class HttpError extends Error {
 }
 
 async function req(url: string, opts: RequestInit = {}): Promise<any> {
-  const r = await fetch(url, {
-    credentials: "include",
-    headers: { "Content-Type": "application/json" },
-    ...opts,
-  });
-  if (!r.ok) throw new HttpError(r.status);
+  const method = opts.method ?? "GET";
+  let r: Response;
+  try {
+    r = await fetch(url, {
+      credentials: "include",
+      headers: { "Content-Type": "application/json" },
+      ...opts,
+    });
+  } catch (e) {
+    // Network/connection failure — surface it in the notification center.
+    notify({
+      level: "error",
+      title: "Network error",
+      message: `Couldn't reach the server (${method} ${url}).`,
+    });
+    throw e;
+  }
+  if (!r.ok) {
+    // Server faults are worth logging; 401/403/404 etc. are handled by callers.
+    if (r.status >= 500) {
+      notify({
+        level: "error",
+        title: `Server error ${r.status}`,
+        message: `${method} ${url}`,
+      });
+    }
+    throw new HttpError(r.status);
+  }
   return r.status === 204 ? null : r.json();
 }