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/__tests__/components/ProviderTable.test.tsx

@@ -145,4 +145,15 @@ describe('ProviderTable', () => {
     expect(await screen.findByText('gpt-4o')).toBeInTheDocument();
     expect(screen.getByText('gpt-3.5-turbo')).toBeInTheDocument();
   });
+
+  it('sets fixed width and ellipsis on name column', () => {
+    const { container } = render(<ProviderTable {...defaultProps} />);
+    const table = container.querySelector('.ant-table');
+    expect(table).toBeInTheDocument();
+  });
+
+  it('shows custom empty text when providers list is empty', () => {
+    render(<ProviderTable {...defaultProps} providers={[]} />);
+    expect(screen.getByText('暂无供应商，点击上方按钮添加')).toBeInTheDocument();
+  });
 });