www/remark42: build frontend with pnpm

• Switch to USES=npm:pnpm so the frontend is built from upstream's pnpm-lock.yaml (frozen, offline, no lifecycle scripts) instead of an npm-resolved dependency set.
• Follow the default Node.js version instead of hardcoding node24.
• Relax the Go requirement to 1.27+.
• Fetch the prefetch distfiles from MASTER_SITES before the framework regenerates them, and recompress the node modules archive with xz.
• Drop the TypeScript patch, no longer needed with upstream's locked dependency versions.
• Remove unused variables and reorder per portclippy.