feat: Add notification bell UI component to header

Implements Phase 1 of notification bell feature with:
- NotificationBell component with badge counter (99+ max)
- useNotifications hook with 30s polling and caching
- Notifications API client for backend integration
- Integration into AppShell header next to user avatar
- Responsive dropdown (400px desktop, full-width mobile)
- Empty state, loading, and error handling
- Optimistic UI updates for mark as read
- Animated badge with pulse effect
- Icon mapping for notification types

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

maternal-web/lib/api/notifications.ts

@@ -0,0 +1,72 @@
+import { apiClient } from './client';
+
+export interface Notification {
+  id: string;
+  userId: string;
+  type: 'feeding' | 'sleep' | 'diaper' | 'medication' | 'milestone' | 'family' | 'system' | 'reminder';
+  title: string;
+  message: string;
+  data?: Record<string, any>;
+  isRead: boolean;
+  isDismissed: boolean;
+  createdAt: string;
+  readAt?: string | null;
+  dismissedAt?: string | null;
+}
+
+export interface GetNotificationsParams {
+  limit?: number;
+  offset?: number;
+  isRead?: boolean;
+  type?: Notification['type'];
+}
+
+export interface GetNotificationsResponse {
+  notifications: Notification[];
+  total: number;
+  unreadCount: number;
+}
+
+export const notificationsApi = {
+  /**
+   * Get user notifications with optional filters
+   */
+  async getNotifications(params?: GetNotificationsParams): Promise<GetNotificationsResponse> {
+    const { data } = await apiClient.get<GetNotificationsResponse>('/notifications', { params });
+    return data;
+  },
+
+  /**
+   * Mark a notification as read
+   */
+  async markAsRead(notificationId: string): Promise<Notification> {
+    const { data } = await apiClient.patch<Notification>(`/notifications/${notificationId}/read`);
+    return data;
+  },
+
+  /**
+   * Mark all notifications as read
+   */
+  async markAllAsRead(): Promise<{ count: number }> {
+    const { data } = await apiClient.patch<{ count: number }>('/notifications/mark-all-read');
+    return data;
+  },
+
+  /**
+   * Dismiss a notification
+   */
+  async dismiss(notificationId: string): Promise<Notification> {
+    const { data } = await apiClient.patch<Notification>(`/notifications/${notificationId}/dismiss`);
+    return data;
+  },
+
+  /**
+   * Get unread notification count
+   */
+  async getUnreadCount(): Promise<number> {
+    const { data } = await apiClient.get<GetNotificationsResponse>('/notifications', {
+      params: { limit: 1, isRead: false },
+    });
+    return data.unreadCount;
+  },
+};