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/AppLayout.test.tsx

@@ -3,8 +3,11 @@ import { describe, it, expect, vi } from 'vitest';
 import { BrowserRouter } from 'react-router';
 import { AppLayout } from '@/components/AppLayout';
 
+const mockToggleTheme = vi.fn();
+const mockSetTheme = vi.fn();
+
 vi.mock('@/contexts/ThemeContext', () => ({
-  useTheme: vi.fn(() => ({ mode: 'light', toggleTheme: vi.fn() })),
+  useTheme: vi.fn(() => ({ mode: 'light', toggleTheme: mockToggleTheme, setTheme: mockSetTheme })),
 }));
 
 const renderWithRouter = (component: React.ReactNode) => {
@@ -26,11 +29,27 @@ describe('AppLayout', () => {
     expect(screen.getByText('用量统计')).toBeInTheDocument();
   });
 
-  it('renders theme toggle button', () => {
+  it('renders theme toggle button with visible color in sidebar', () => {
     renderWithRouter(<AppLayout />);
 
     const themeButton = screen.getByRole('button', { name: 'moon' });
     expect(themeButton).toBeInTheDocument();
+    expect(themeButton.style.color).toBe('rgba(255, 255, 255, 0.85)');
+  });
+
+  it('renders theme toggle button visible in dark mode', async () => {
+    const { useTheme } = await import('@/contexts/ThemeContext');
+    vi.mocked(useTheme).mockReturnValue({
+      mode: 'dark',
+      toggleTheme: mockToggleTheme,
+      setTheme: mockSetTheme,
+    });
+
+    renderWithRouter(<AppLayout />);
+
+    const themeButton = screen.getByRole('button', { name: 'sun' });
+    expect(themeButton).toBeInTheDocument();
+    expect(themeButton.style.color).toBe('rgba(255, 255, 255, 0.85)');
   });
 
   it('renders content outlet', () => {