feat: implement dynamic changelog loading from API

- Add GET /api/changelog endpoint to fetch changelog from CHANGELOG.md
- Create service/changelog.go to parse CHANGELOG.md markdown file
- Add handler/changelog.go to handle changelog requests
- Update ChangelogDrawer component to fetch from API instead of hardcoded data
- Export apiFetch from lib/api.ts for public use
- Add changelog parser tests with 14 version entries verified

server/router/router.go

@@ -46,6 +46,9 @@ func setupAPIRoutes(r *gin.Engine) {
 		api.POST("/auth/login", handler.Login)
 		api.GET("/auth/validate", handler.ValidateToken)
 
+		// 公开接口（无需登录）
+		api.GET("/changelog", handler.GetChangelog)
+
 		// 需要登录的 API
 		authed := api.Group("/")
 		authed.Use(middleware.AuthRequired())