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__/utils/version.test.ts

@@ -0,0 +1,21 @@
+import { describe, it, expect } from 'vitest'
+import { getVersionStatus } from '@/utils/version'
+
+describe('getVersionStatus', () => {
+  it('returns matched when versions are equal', () => {
+    expect(getVersionStatus('1.2.3', { version: '1.2.3', commit: 'abc', buildTime: 'now' }).kind).toBe('matched')
+  })
+
+  it('returns mismatched when release versions differ', () => {
+    expect(getVersionStatus('1.2.3', { version: '1.2.4', commit: 'abc', buildTime: 'now' }).kind).toBe('mismatched')
+  })
+
+  it('returns unknown for dev or unknown versions', () => {
+    expect(getVersionStatus('dev', { version: '1.2.3', commit: 'abc', buildTime: 'now' }).kind).toBe('unknown')
+    expect(getVersionStatus('1.2.3', { version: 'unknown', commit: 'abc', buildTime: 'now' }).kind).toBe('unknown')
+  })
+
+  it('returns unavailable on request failure', () => {
+    expect(getVersionStatus('1.2.3', undefined, true).kind).toBe('unavailable')
+  })
+})