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__/api/version.test.ts

@@ -0,0 +1,30 @@
+import { http, HttpResponse } from 'msw'
+import { setupServer } from 'msw/node'
+import { describe, it, expect, beforeAll, afterEach, afterAll } from 'vitest'
+import { getBackendVersion } from '@/api/version'
+
+describe('version API', () => {
+  const server = setupServer()
+
+  beforeAll(() => server.listen({ onUnhandledRequest: 'bypass' }))
+  afterEach(() => server.resetHandlers())
+  afterAll(() => server.close())
+
+  it('fetches backend version and converts build_time to buildTime', async () => {
+    server.use(
+      http.get('http://localhost:3000/api/version', () => {
+        return HttpResponse.json({
+          version: '0.1.0',
+          commit: 'abc1234',
+          build_time: '2026-05-05T00:00:00Z',
+        })
+      })
+    )
+
+    await expect(getBackendVersion()).resolves.toEqual({
+      version: '0.1.0',
+      commit: 'abc1234',
+      buildTime: '2026-05-05T00:00:00Z',
+    })
+  })
+})