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__/hooks/useVersion.test.tsx

@@ -0,0 +1,33 @@
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import { renderHook, waitFor } from '@testing-library/react'
+import { http, HttpResponse } from 'msw'
+import { setupServer } from 'msw/node'
+import React from 'react'
+import { useBackendVersion } from '@/hooks/useVersion'
+
+const server = setupServer(
+  http.get('/api/version', () => {
+    return HttpResponse.json({ version: '0.1.0', commit: 'abc1234', build_time: '2026-05-05T00:00:00Z' })
+  })
+)
+
+function createWrapper() {
+  const queryClient = new QueryClient({ defaultOptions: { queries: { retry: false } } })
+  return function Wrapper({ children }: { children: React.ReactNode }) {
+    return <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+  }
+}
+
+beforeAll(() => server.listen())
+afterEach(() => server.resetHandlers())
+afterAll(() => server.close())
+
+describe('useBackendVersion', () => {
+  it('fetches backend version', async () => {
+    const { result } = renderHook(() => useBackendVersion(), { wrapper: createWrapper() })
+
+    await waitFor(() => expect(result.current.isSuccess).toBe(true))
+
+    expect(result.current.data).toEqual({ version: '0.1.0', commit: 'abc1234', buildTime: '2026-05-05T00:00:00Z' })
+  })
+})