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/api/notifications.ts

@@ -0,0 +1,64 @@
+import { apiClient } from "./client";
+
+export interface NotificationItem {
+	id: string;
+	user_id: string;
+	category: string;
+	severity: "info" | "warning" | "error" | "success";
+	title: string;
+	message: string | null;
+	source_type: string | null;
+	source_id: string | null;
+	metadata: Record<string, unknown>;
+	read_at: string | null;
+	dismissed_at: string | null;
+	created_at: string;
+}
+
+export interface NotificationListResponse {
+	items: NotificationItem[];
+	total: number;
+	limit: number;
+	offset: number;
+}
+
+export interface UnreadCountResponse {
+	count: number;
+}
+
+export interface MarkAllReadResponse {
+	marked_count: number;
+}
+
+export const getNotifications = async (): Promise<NotificationListResponse> => {
+	const response =
+		await apiClient.get<NotificationListResponse>("/notifications");
+	return response.data;
+};
+
+export const getUnreadCount = async (): Promise<number> => {
+	const response = await apiClient.get<UnreadCountResponse>(
+		"/notifications/unread",
+	);
+	return response.data.count;
+};
+
+export const markNotificationRead = async (
+	id: string,
+): Promise<NotificationItem> => {
+	const response = await apiClient.patch<NotificationItem>(
+		`/notifications/${id}/read`,
+	);
+	return response.data;
+};
+
+export const markAllNotificationsRead = async (): Promise<number> => {
+	const response = await apiClient.post<MarkAllReadResponse>(
+		"/notifications/mark-all-read",
+	);
+	return response.data.marked_count;
+};
+
+export const dismissNotification = async (id: string): Promise<void> => {
+	await apiClient.delete(`/notifications/${id}`);
+};