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/Feed.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState } from "react";
 import { useInfiniteQuery, useQuery, useQueryClient } from "@tanstack/react-query";
 import { api, type FeedFilters, type Video } from "../lib/api";
-import { toast } from "../lib/toast";
+import { toast } from "../lib/notifications";
 import VideoCard from "./VideoCard";
 
 const PAGE = 60;