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/desktop/main.go

@@ -130,6 +130,7 @@ func main() {
 	providerHandler := handler.NewProviderHandler(providerService)
 	modelHandler := handler.NewModelHandler(modelService)
 	statsHandler := handler.NewStatsHandler(statsService)
+	versionHandler := handler.NewVersionHandler()
 
 	gin.SetMode(gin.ReleaseMode)
 	r := gin.New()
@@ -139,7 +140,7 @@ func main() {
 	r.Use(middleware.Logging(zapLogger))
 	r.Use(middleware.CORS())
 
-	setupRoutes(r, proxyHandler, providerHandler, modelHandler, statsHandler)
+	setupRoutes(r, proxyHandler, providerHandler, modelHandler, statsHandler, versionHandler)
 	setupStaticFiles(r)
 
 	server = &http.Server{
@@ -172,9 +173,10 @@ func main() {
 	setupSystray(port)
 }
 
-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("/openai/*path", withProtocol("openai", proxyHandler.HandleProxy))
 	r.Any("/anthropic/*path", withProtocol("anthropic", proxyHandler.HandleProxy))
+	r.GET("/api/version", versionHandler.GetVersion)
 
 	providers := r.Group("/api/providers")
 	{
@@ -257,13 +259,13 @@ func setupStaticFilesWithFS(r *gin.Engine, distFS fs.FS) {
 		c.Data(200, getContentType(filepath), data)
 	})
 
-	r.GET("/favicon.svg", func(c *gin.Context) {
-		data, err := fs.ReadFile(distFS, "favicon.svg")
+	r.GET("/icon.png", func(c *gin.Context) {
+		data, err := fs.ReadFile(distFS, "icon.png")
 		if err != nil {
 			c.Status(404)
 			return
 		}
-		c.Data(200, "image/svg+xml", data)
+		c.Data(200, "image/png", data)
 	})
 
 	r.NoRoute(func(c *gin.Context) {