style: 修复前端 UI 细节问题

- 供应商/模型/统计表格增加列宽约束和 ellipsis 省略号
- 修复主题切换按钮在暗色侧边栏中不可见
- 表格添加 scroll 属性防止窄屏溢出
- 自定义表格空状态提示文案

frontend/src/pages/Providers/ProviderTable.tsx

@@ -1,4 +1,4 @@
-import { Button, Table, Tag, Popconfirm, Space, Card } from 'antd';
+import { Button, Table, Tag, Popconfirm, Space, Card, Tooltip } from 'antd';
 import type { ColumnsType } from 'antd/es/table';
 import type { Provider, Model } from '@/types';
 import { ModelTable } from './ModelTable';
@@ -33,17 +33,26 @@ export function ProviderTable({
       title: '名称',
       dataIndex: 'name',
       key: 'name',
+      width: 180,
+      ellipsis: { showTitle: true },
     },
     {
       title: 'Base URL',
       dataIndex: 'baseUrl',
       key: 'baseUrl',
+      ellipsis: { showTitle: true },
     },
     {
       title: 'API Key',
       dataIndex: 'apiKey',
       key: 'apiKey',
-      render: (key: string | null | undefined) => maskApiKey(key),
+      width: 120,
+      ellipsis: { showTitle: true },
+      render: (key: string | null | undefined) => (
+        <Tooltip title={maskApiKey(key)}>
+          <span>{maskApiKey(key)}</span>
+        </Tooltip>
+      ),
     },
     {
       title: '状态',
@@ -101,6 +110,8 @@ export function ProviderTable({
           ),
         }}
         pagination={false}
+        scroll={{ x: 840 }}
+        locale={{ emptyText: '暂无供应商，点击上方按钮添加' }}
       />
     </Card>
   );