chore: upgrade Go version to 1.26

- backend/go.mod: go 1.26
- deploy/docker/Dockerfile: golang:1.26-alpine
- deploy/docker/Dockerfile.cn: golang:1.26-alpine

deploy/docker/Dockerfile.cn

@@ -13,7 +13,7 @@ RUN npm config set registry https://registry.npmmirror.com \
 
 # 后端：go.mod 使用纯 Go 的 glebarez/sqlite，可关闭 CGO 直接交叉编译到目标架构，
 # 因此同样固定在原生平台构建
-FROM --platform=$BUILDPLATFORM golang:1.23-alpine AS backend
+FROM --platform=$BUILDPLATFORM golang:1.26-alpine AS backend
 LABEL author="github.com/Sakurasan"
 ARG TARGETOS
 ARG TARGETARCH