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/components/NotificationPanel.tsx

@@ -0,0 +1,76 @@
+import { useEffect } from 'react';
+import { Badge, List, Popover, Button, Empty, Typography, Space } from 'antd';
+import { BellOutlined } from '@ant-design/icons';
+import { useNavigate } from 'react-router-dom';
+import { useMessageStore } from '../stores/message';
+
+const { Text } = Typography;
+
+export default function NotificationPanel() {
+  const navigate = useNavigate();
+  const { unreadCount, recentMessages, fetchUnreadCount, fetchRecentMessages, markAsRead } =
+    useMessageStore();
+
+  useEffect(() => {
+    fetchUnreadCount();
+    fetchRecentMessages();
+    // 每 60 秒刷新一次
+    const interval = setInterval(() => {
+      fetchUnreadCount();
+      fetchRecentMessages();
+    }, 60000);
+    return () => clearInterval(interval);
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []);
+
+  const content = (
+    <div style={{ width: 360 }}>
+      {recentMessages.length === 0 ? (
+        <Empty description="暂无消息" image={Empty.PRESENTED_IMAGE_SIMPLE} />
+      ) : (
+        <List
+          dataSource={recentMessages}
+          renderItem={(item) => (
+            <List.Item
+              style={{ padding: '8px 0', cursor: 'pointer' }}
+              onClick={() => {
+                if (!item.is_read) {
+                  markAsRead(item.id);
+                }
+              }}
+            >
+              <List.Item.Meta
+                title={
+                  <Space>
+                    <Text strong={!item.is_read} ellipsis style={{ maxWidth: 260 }}>
+                      {item.title}
+                    </Text>
+                    {!item.is_read && <span style={{ display: 'inline-block', width: 6, height: 6, borderRadius: '50%', background: '#1677ff' }} />}
+                  </Space>
+                }
+                description={
+                  <Text type="secondary" ellipsis style={{ maxWidth: 300 }}>
+                    {item.body}
+                  </Text>
+                }
+              />
+            </List.Item>
+          )}
+        />
+      )}
+      <div style={{ textAlign: 'center', paddingTop: 8, borderTop: '1px solid #f0f0f0' }}>
+        <Button type="link" onClick={() => navigate('/messages')}>
+          查看全部消息
+        </Button>
+      </div>
+    </div>
+  );
+
+  return (
+    <Popover content={content} trigger="click" placement="bottomRight">
+      <Badge count={unreadCount} size="small" offset={[4, -4]}>
+        <BellOutlined style={{ fontSize: 18, cursor: 'pointer' }} />
+      </Badge>
+    </Popover>
+  );
+}