feat: 统一品牌标识、关于页面三卡片布局与版本诊断功能

- 统一品牌为 Nex：侧边栏、托盘 tooltip、HTML 标题、favicon (PNG 替代 SVG)
- 重构关于页面为三卡片布局（品牌/版本/链接），版本状态 Tag 绝对定位右上角
- 新增 GET /api/version 后端接口，返回 version/commit/build_time
- 新增前端版本一致性诊断：匹配/不匹配/不可判断三种状态
- 同步 delta specs 到主 specs 并归档变更

frontend/src/__tests__/components/AppLayout.test.tsx

@@ -1,25 +1,37 @@
 import { render, screen } from '@testing-library/react'
-import { BrowserRouter } from 'react-router'
+import userEvent from '@testing-library/user-event'
+import { MemoryRouter } from 'react-router'
 import { describe, it, expect } from 'vitest'
 import { AppLayout } from '@/components/AppLayout'
 
 const renderWithRouter = (component: React.ReactNode) => {
-  return render(<BrowserRouter>{component}</BrowserRouter>)
+  return render(<MemoryRouter initialEntries={['/providers']}>{component}</MemoryRouter>)
 }
 
 describe('AppLayout', () => {
   it('renders sidebar with app name', () => {
     renderWithRouter(<AppLayout />)
 
-    const appNames = screen.getAllByText('AI Gateway')
-    expect(appNames.length).toBeGreaterThan(0)
+    expect(screen.getByText('Nex')).toBeInTheDocument()
+    expect(screen.getByAltText('Nex logo')).toBeInTheDocument()
+  })
+
+  it('keeps logo visible when sidebar is collapsed', async () => {
+    const user = userEvent.setup()
+    renderWithRouter(<AppLayout />)
+
+    await user.click(screen.getByLabelText('收起侧边栏'))
+
+    expect(screen.getByAltText('Nex logo')).toBeInTheDocument()
+    expect(screen.queryByText('Nex')).not.toBeInTheDocument()
+    expect(screen.getByLabelText('展开侧边栏')).toBeInTheDocument()
   })
 
   it('renders navigation menu items', () => {
     renderWithRouter(<AppLayout />)
 
-    expect(screen.getByText('供应商管理')).toBeInTheDocument()
-    expect(screen.getByText('用量统计')).toBeInTheDocument()
+    expect(screen.getAllByText('供应商管理').length).toBeGreaterThan(0)
+    expect(screen.getAllByText('用量统计').length).toBeGreaterThan(0)
   })
 
   it('renders settings menu item', () => {
@@ -28,6 +40,12 @@ describe('AppLayout', () => {
     expect(screen.getByText('设置')).toBeInTheDocument()
   })
 
+  it('renders about menu item', () => {
+    renderWithRouter(<AppLayout />)
+
+    expect(screen.getByText('关于')).toBeInTheDocument()
+  })
+
   it('renders content outlet', () => {
     const { container } = renderWithRouter(<AppLayout />)