fix(server): bump Dockerfile to golang:1.26-alpine (matches go.mod 1.26.3)

Backend v2 (merged from upstream/main) bumped go.mod to 1.26.3, but the
multi-stage Dockerfile (PR m5stack#19) still pinned golang:1.24-alpine. Build
was failing on 'go mod download' with GOTOOLCHAIN=local.

Author: Flo5k5

server/manifest/docker/Dockerfile

@@ -9,7 +9,7 @@
 #     context: https://github.com/m5stack/StackChan.git#main:server
 #     dockerfile: manifest/docker/Dockerfile
 
-FROM golang:1.24-alpine AS builder
+FROM golang:1.26-alpine AS builder
 
 RUN apk add --no-cache gcc musl-dev sqlite-dev