Enable Vercel image optimization (drop static export mode)

[apbendi]

Currently in next.config.js:

output: "export",
images: { unoptimized: true },

Static-export mode disables Next.js's <Image> optimization pipeline. Every image in public/images/ is served at original size and format — no responsive sizing, no WebP/AVIF, no lazy-load. Several pages have heavy PNG screenshots that would benefit.

What's needed

1. Drop output: "export" and images: { unoptimized: true } from next.config.js.
2. Update the PR check workflow (.github/workflows/pr-check.yml). The link checker (lychee) runs against ./out/**/*.html, which only exists in static-export mode. Pick one:
   • Add a separate "static export for link check" script (e.g. pnpm build:export calling next build with output: "export" enabled just for that step), or
   • Switch link checker to one that reads MDX sources directly, or
   • Drop the link check from CI and rely on Vercel preview deploys for manual verification.
3. Confirm pnpm build produces a .next/ directory locally and Vercel still picks it up automatically.

Why this is on hold

When we switched from GitHub Pages to Vercel hosting, keeping static export was the minimum-change option — it preserved the existing link-check tooling. Enabling image optimization is a separate, optional improvement.

Verification

• Build succeeds without output: "export"
• Vercel preview deploy renders all images correctly, in WebP/AVIF where supported
• PR link check still runs (in whatever new form is chosen)
• No content-image regressions (compare some heavy pages like tally-features/enterprise-features/notifications)