go: move to 1.27 (toolchain 1.27.1)

go.mod and the Dockerfile builder go from 1.25 to 1.27. The toolchain line
pins 1.27.1 so the server's auto download grabs the patch release, not 1.27.0.
tidy also moved chehsunliu/poker into the direct requires, where it belonged.

Dockerfile

@@ -6,7 +6,7 @@ COPY tailwind.config.js ./
 COPY internal/web ./internal/web
 RUN npx tailwindcss -i internal/web/static/css/input.css -o internal/web/static/css/output.css --minify
 
-FROM golang:1.25-alpine AS builder
+FROM golang:1.27-alpine AS builder
 WORKDIR /app
 COPY go.mod go.sum ./
 RUN go mod download