ci: add GitHub Actions CI workflow (lint, typecheck, build)

[teckedd-code2save]

Summary

Add a GitHub Actions CI workflow that runs lint + type checking + production build on every push and PR. The repo currently has no .github/ directory at all — changes to this Next.js 16 / TypeScript / Tailwind 4 site ship with zero automated checks.

What to do

• Create .github/workflows/ci.yml
• Trigger: push to main and pull_request
• Job steps (Node 22, npm ci with cache):
  1. npm run lint (ESLint config already exists: eslint.config.mjs)
  2. npx tsc --noEmit (or the repo's typecheck script)
  3. npm run build — Next.js static prerender (next build); the site is statically exported so this is safe in CI
• Keep it a single workflow for simplicity, matching the style of groundcontrol/.github/workflows/ci.yml

Why it matters

• The site is the first thing users see before installing GroundControl on a VPS — a broken build or lint error reaching production damages trust
• GroundControl core repo has CI (ci/deploy/test workflows); its product site should too
• Catches TypeScript regressions and broken prerenders before deploy to Vercel

Acceptance criteria

• .github/workflows/ci.yml exists
• Workflow passes on the current main (green checkmark on the next push)
• Lint, typecheck, and build all run as separate steps
• A failing change blocks merge (status check on PRs)