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/handler/changelog.go

@@ -0,0 +1,26 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"billai-server/service"
+)
+
+// GetChangelog GET /api/changelog 获取版本变更日志
+func GetChangelog(c *gin.Context) {
+	changelog, err := service.ParseChangelog()
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{
+			"result":  false,
+			"message": "获取变更日志失败: " + err.Error(),
+		})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"result": true,
+		"data":   changelog,
+	})
+}