style: 优化前端样式，提升现代化设计感

- ConfigProvider 注入全局配置（动画、表格尺寸）
- CSS Variables 主题微调（页面背景、圆角、字体栈）
- AppLayout Menu 支持 logo/operations/collapsed
- Statistic 组件增加 color/prefix/suffix/animation
- Card 组件启用 hoverShadow/headerBordered
- Table 组件启用 stripe 斑马纹
- Tag 组件使用 variant="light" + shape="round"
- Dialog 居中显示并设置固定宽度
- 布局样式硬编码颜色替换为 TDesign Token
- UsageChart 改用 AreaChart + 渐变填充
- 更新 frontend spec 同步样式体系要求

frontend/src/pages/Stats/StatCards.tsx

@@ -1,4 +1,5 @@
 import { Row, Col, Card, Statistic } from 'tdesign-react';
+import { ChartBarIcon, ChartLineIcon, ServerIcon, Calendar1Icon } from 'tdesign-icons-react';
 import type { UsageStats } from '@/types';
 
 interface StatCardsProps {
@@ -18,23 +19,55 @@ export function StatCards({ stats }: StatCardsProps) {
   return (
     <Row gutter={[16, 16]} style={{ marginBottom: 16 }}>
       <Col xs={12} md={6}>
-        <Card>
-          <Statistic title="总请求量" value={totalRequests} />
+        <Card bordered={false} hoverShadow>
+          <Statistic
+            title="总请求量"
+            value={totalRequests}
+            color="blue"
+            prefix={<ChartBarIcon />}
+            suffix="次"
+            animation={{ duration: 800, valueFrom: 0 }}
+            animationStart
+          />
         </Card>
       </Col>
       <Col xs={12} md={6}>
-        <Card>
-          <Statistic title="活跃模型数" value={activeModels} />
+        <Card bordered={false} hoverShadow>
+          <Statistic
+            title="活跃模型数"
+            value={activeModels}
+            color="green"
+            prefix={<ChartLineIcon />}
+            suffix="个"
+            animation={{ duration: 800, valueFrom: 0 }}
+            animationStart
+          />
         </Card>
       </Col>
       <Col xs={12} md={6}>
-        <Card>
-          <Statistic title="活跃供应商数" value={activeProviders} />
+        <Card bordered={false} hoverShadow>
+          <Statistic
+            title="活跃供应商数"
+            value={activeProviders}
+            color="orange"
+            prefix={<ServerIcon />}
+            suffix="个"
+            animation={{ duration: 800, valueFrom: 0 }}
+            animationStart
+          />
         </Card>
       </Col>
       <Col xs={12} md={6}>
-        <Card>
-          <Statistic title="今日请求量" value={todayRequests} />
+        <Card bordered={false} hoverShadow>
+          <Statistic
+            title="今日请求量"
+            value={todayRequests}
+            color="red"
+            prefix={<Calendar1Icon />}
+            suffix="次"
+            animation={{ duration: 800, valueFrom: 0 }}
+            animationStart
+          />
         </Card>
       </Col>
     </Row>