feat: notification center frontend core (PR-3)

- Bell icon in icon registry (Phosphor Bell)
- NotificationProvider context with polling (15s unread / 30s list)
- useNotifications() hook with optimistic updates
- NotificationCenter component: bell + badge + dropdown panel
- NotificationItem component: severity icon, title, relative time, actions
- AppShell integration: mount in header-actions, hidden on mobile
- CSS styles: dropdown, items, unread/read states, empty state
- formatRelativeTime utility (custom, no new deps)
- 25 frontend tests (9 hook + 6 item + 10 center)

Quality gates: vitest 25 passed, tsc clean, eslint clean

apps/web/src/hooks/use-notifications.ts

@@ -0,0 +1,12 @@
+import { useContext } from "react";
+import { NotificationContext } from "../state/notifications";
+
+export function useNotifications() {
+	const ctx = useContext(NotificationContext);
+	if (!ctx) {
+		throw new Error(
+			"useNotifications must be used within NotificationProvider",
+		);
+	}
+	return ctx;
+}