feat: create StatisticsPlaceholder component

Add placeholder component for future statistics area with 60vh height.

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

src/pages/components/StatisticsPlaceholder.tsx

@@ -0,0 +1,29 @@
+import React from 'react';
+
+const StatisticsPlaceholder: React.FC = () => {
+  return (
+    <div
+      style={{
+        minHeight: '60vh',
+        background: '#f0f2f5',
+        padding: '24px',
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'center',
+      }}
+    >
+      {/* 未来扩展：统计数据卡片、图表、报表等 */}
+      <div
+        style={{
+          color: '#bfbfbf',
+          fontSize: '16px',
+          textAlign: 'center',
+        }}
+      >
+        {/* 当前阶段：完全空白，便于未来扩展 */}
+      </div>
+    </div>
+  );
+};
+
+export default StatisticsPlaceholder;