ci: 修复前端镜像仓库登录凭据

将前端 workflow 的容器仓库登录凭据改为与后端一致的 GIT_USERNAME/GIT_TOKEN，避免登录步骤缺少用户名或密码导致失败。

Made-with: Cursor

.gitea/workflows/build.yml

@@ -144,8 +144,8 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ secrets.REGISTRY_USERNAME }}
-          password: ${{ secrets.REGISTRY_PASSWORD }}
+          username: ${{ secrets.GIT_USERNAME }}
+          password: ${{ secrets.GIT_TOKEN }}
 
       - name: Extract metadata for Docker
         id: meta