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

backend/cmd/server/main.go

@@ -93,6 +93,7 @@ func main() {
 	providerHandler := handler.NewProviderHandler(providerService)
 	modelHandler := handler.NewModelHandler(modelService)
 	statsHandler := handler.NewStatsHandler(statsService)
+	versionHandler := handler.NewVersionHandler()
 
 	gin.SetMode(gin.ReleaseMode)
 	r := gin.New()
@@ -102,7 +103,7 @@ func main() {
 	r.Use(middleware.Logging(zapLogger))
 	r.Use(middleware.CORS())
 
-	setupRoutes(r, proxyHandler, providerHandler, modelHandler, statsHandler)
+	setupRoutes(r, proxyHandler, providerHandler, modelHandler, statsHandler, versionHandler)
 
 	srv := &http.Server{
 		Addr:         fmt.Sprintf(":%d", cfg.Server.Port),
@@ -140,8 +141,9 @@ func main() {
 	zapLogger.Info("服务器已关闭")
 }
 
-func setupRoutes(r *gin.Engine, proxyHandler *handler.ProxyHandler, providerHandler *handler.ProviderHandler, modelHandler *handler.ModelHandler, statsHandler *handler.StatsHandler) {
+func setupRoutes(r *gin.Engine, proxyHandler *handler.ProxyHandler, providerHandler *handler.ProviderHandler, modelHandler *handler.ModelHandler, statsHandler *handler.StatsHandler, versionHandler *handler.VersionHandler) {
 	r.Any("/:protocol/*path", proxyHandler.HandleProxy)
+	r.GET("/api/version", versionHandler.GetVersion)
 
 	providers := r.Group("/api/providers")
 	{