fix: upgrade Node to 24 so the Docker build works with sharp 0.35

sharp 0.35.5 requires Node >=20.9.0, so the node:18-alpine image failed
to load the linuxmusl-arm64 binary and the production build broke.

Move the image and both CI workflows to Node 24 (current Active LTS),
raise the engines floor to the version sharp actually needs, and declare
packageManager to silence the corepack warning.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:18-alpine AS base
+FROM node:24-alpine AS base
 
 RUN apk add --no-cache libc6-compat
 RUN corepack enable && corepack prepare [email protected] --activate