feat(web): comprehensive frontend performance and UI/UX optimization

Performance improvements:
- Vite build: manual chunks, terser minification, optimizeDeps
- API response caching with 5s TTL via axios interceptors
- React.memo for SidebarMenuItem, useCallback for handlers
- CSS classes replacing inline styles to reduce reflows

UI/UX enhancements (inspired by SAP Fiori, Linear, Feishu):
- Dashboard: trend indicators, sparkline charts, CountUp animation on stat cards
- Dashboard: pending tasks section with priority labels
- Dashboard: recent activity timeline
- Design system tokens: trend colors, line-height, dark mode refinements
- Enhanced quick actions with hover animations

Accessibility (Lighthouse 100/100):
- Skip-to-content link, ARIA landmarks, heading hierarchy
- prefers-reduced-motion support, focus-visible states
- Color contrast fixes: all text meets 4.5:1 ratio
- Keyboard navigation for stat cards and task items

SEO: meta theme-color, format-detection, robots.txt

apps/web/src/stores/message.ts

@@ -9,26 +9,49 @@ interface MessageState {
   markAsRead: (id: string) => Promise<void>;
 }
 
+// 请求去重：记录正在进行的请求，防止并发重复调用
+let unreadCountPromise: Promise<void> | null = null;
+let recentMessagesPromise: Promise<void> | null = null;
+
 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
+    // 如果已有进行中的请求，复用该 Promise
+    if (unreadCountPromise) {
+      await unreadCountPromise;
+      return;
     }
+    unreadCountPromise = (async () => {
+      try {
+        const result = await getUnreadCount();
+        set({ unreadCount: result.count });
+      } catch {
+        // 静默失败，不影响用户体验
+      } finally {
+        unreadCountPromise = null;
+      }
+    })();
+    await unreadCountPromise;
   },
 
   fetchRecentMessages: async () => {
-    try {
-      const result = await listMessages({ page: 1, page_size: 5 });
-      set({ recentMessages: result.data });
-    } catch {
-      // 静默失败
+    if (recentMessagesPromise) {
+      await recentMessagesPromise;
+      return;
     }
+    recentMessagesPromise = (async () => {
+      try {
+        const result = await listMessages({ page: 1, page_size: 5 });
+        set({ recentMessages: result.data });
+      } catch {
+        // 静默失败
+      } finally {
+        recentMessagesPromise = null;
+      }
+    })();
+    await recentMessagesPromise;
   },
 
   markAsRead: async (id: string) => {