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/stores/message.ts

@@ -0,0 +1,47 @@
+import { create } from 'zustand';
+import { getUnreadCount, listMessages, markRead, type MessageInfo } from '../api/messages';
+
+interface MessageState {
+  unreadCount: number;
+  recentMessages: MessageInfo[];
+  fetchUnreadCount: () => Promise<void>;
+  fetchRecentMessages: () => Promise<void>;
+  markAsRead: (id: string) => Promise<void>;
+}
+
+export const useMessageStore = create<MessageState>((set) => ({
+  unreadCount: 0,
+  recentMessages: [],
+
+  fetchUnreadCount: async () => {
+    try {
+      const result = await getUnreadCount();
+      set({ unreadCount: result.count });
+    } catch {
+      // 静默失败，不影响用户体验
+    }
+  },
+
+  fetchRecentMessages: async () => {
+    try {
+      const result = await listMessages({ page: 1, page_size: 5 });
+      set({ recentMessages: result.data });
+    } catch {
+      // 静默失败
+    }
+  },
+
+  markAsRead: async (id: string) => {
+    try {
+      await markRead(id);
+      set((state) => ({
+        unreadCount: Math.max(0, state.unreadCount - 1),
+        recentMessages: state.recentMessages.map((m) =>
+          m.id === id ? { ...m, is_read: true } : m,
+        ),
+      }));
+    } catch {
+      // 静默失败
+    }
+  },
+}));