feat(message): add message center module (Phase 5)

Implement the complete message center with:
- Database migrations for message_templates, messages, message_subscriptions tables
- erp-message crate with entities, DTOs, services, handlers
- Message CRUD, send, read/unread tracking, soft delete
- Template management with variable interpolation
- Subscription preferences with DND support
- Frontend: messages page, notification panel, unread count badge
- Server integration with module registration and routing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/layouts/MainLayout.tsx

@@ -1,17 +1,18 @@
 import { Layout, Menu, theme, Avatar, Space, Dropdown, Button } from 'antd';
+import NotificationPanel from '../components/NotificationPanel';
 import {
   HomeOutlined,
   UserOutlined,
   SafetyOutlined,
   ApartmentOutlined,
-  BellOutlined,
   SettingOutlined,
   MenuFoldOutlined,
   MenuUnfoldOutlined,
   PartitionOutlined,
   LogoutOutlined,
+  MessageOutlined,
 } from '@ant-design/icons';
-import { useNavigate } from 'react-router-dom';
+import { useNavigate, useLocation } from 'react-router-dom';
 import { useAppStore } from '../stores/app';
 import { useAuthStore } from '../stores/auth';
 
@@ -23,6 +24,7 @@ const menuItems = [
   { key: '/roles', icon: <SafetyOutlined />, label: '权限管理' },
   { key: '/organizations', icon: <ApartmentOutlined />, label: '组织架构' },
   { key: '/workflow', icon: <PartitionOutlined />, label: '工作流' },
+  { key: '/messages', icon: <MessageOutlined />, label: '消息中心' },
   { key: '/settings', icon: <SettingOutlined />, label: '系统设置' },
 ];
 
@@ -31,6 +33,8 @@ export default function MainLayout({ children }: { children: React.ReactNode })
   const { user, logout } = useAuthStore();
   const { token } = theme.useToken();
   const navigate = useNavigate();
+  const location = useLocation();
+  const currentPath = location.pathname || '/';
 
   const userMenuItems = [
     {
@@ -64,7 +68,7 @@ export default function MainLayout({ children }: { children: React.ReactNode })
           theme="dark"
           mode="inline"
           items={menuItems}
-          defaultSelectedKeys={['/']}
+          selectedKeys={[currentPath]}
           onClick={({ key }) => navigate(key)}
         />
       </Sider>
@@ -87,7 +91,7 @@ export default function MainLayout({ children }: { children: React.ReactNode })
             />
           </Space>
           <Space size="middle">
-            <BellOutlined style={{ fontSize: 18, cursor: 'pointer' }} />
+            <NotificationPanel />
             <Dropdown menu={{ items: userMenuItems }} placement="bottomRight">
               <Space style={{ cursor: 'pointer' }}>
                 <Avatar icon={<UserOutlined />} />