feat: 添加消息中心及顶栏未读角标功能

新增消息中心页面，实现未读消息轮询与展示
为各 Tab 页面顶栏添加通知图标及未读数角标
新增通知 API 接口与未读数状态管理
实现消息标记已读、全部已读及下拉刷新功能

lib/shared/widgets/x_top_bar.dart

@@ -26,6 +26,7 @@ class XTopBar extends StatelessWidget {
     String trailingIcon = 'notification',
     Color trailingColor = AppColors.onSurfaceVariant,
     VoidCallback? onTrailingTap,
+    int trailingBadge = 0,
   }) {
     return XTopBar._(
       title: title,
@@ -46,7 +47,18 @@ class XTopBar extends StatelessWidget {
       trailing: GestureDetector(
         onTap: onTrailingTap,
         behavior: HitTestBehavior.opaque,
-        child: XIcon(trailingIcon, size: 22, color: trailingColor),
+        child: Stack(
+          clipBehavior: Clip.none,
+          children: [
+            XIcon(trailingIcon, size: 22, color: trailingColor),
+            if (trailingBadge > 0)
+              Positioned(
+                top: -6,
+                right: -8,
+                child: _TrailingBadge(count: trailingBadge),
+              ),
+          ],
+        ),
       ),
       titleStyle: AppTypography.headlineMdMobile,
       titleAlign: TextAlign.left,
@@ -135,4 +147,35 @@ class XTopBar extends StatelessWidget {
       ),
     );
   }
+}
+
+/// 顶栏图标右上角的未读角标：红底白字胶囊，超过 99 显示 `99+`。
+class _TrailingBadge extends StatelessWidget {
+  const _TrailingBadge({required this.count});
+
+  final int count;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      constraints: const BoxConstraints(minWidth: 16),
+      height: 16,
+      padding: const EdgeInsets.symmetric(horizontal: 4),
+      alignment: Alignment.center,
+      decoration: BoxDecoration(
+        color: AppColors.statusError,
+        borderRadius: BorderRadius.circular(9999),
+        border: Border.all(color: AppColors.surface, width: 1.5),
+      ),
+      child: Text(
+        count > 99 ? '99+' : '$count',
+        style: const TextStyle(
+          fontSize: 10,
+          height: 1,
+          fontWeight: FontWeight.w700,
+          color: AppColors.surface,
+        ),
+      ),
+    );
+  }
 }