Add esbuild minification of editor.js in Docker build

Reduces editor.js from 1.1MB to 591KB (48%) using a separate
node:alpine build stage with npx esbuild --minify.

Co-Authored-By: Claude Sonnet 4.6 <[e-mail]>

Dockerfile

···
+FROM node:alpine AS jsbuilder
+
+WORKDIR /build
+COPY static/vendor/editor.js ./editor.js
+RUN npx --yes esbuild editor.js --bundle=false --minify --outfile=editor.min.js
+
 FROM golang:1.23-alpine AS builder
 
 RUN apk add --no-cache gcc musl-dev
···
 COPY --from=builder /app/markdownhub .
 COPY --from=builder /app/templates ./templates
 COPY --from=builder /app/static ./static
+COPY --from=jsbuilder /build/editor.min.js ./static/vendor/editor.js
 COPY --from=builder /app/migrations ./migrations
 
 EXPOSE 8080