Add Lighthouse CI workflow to audit the page (closes #8)

[glebfox]

Summary

Adds a GitHub Actions workflow that audits the page with Lighthouse CI on every push to master and on PRs targeting it (closes #8). The page is static and dependency-free, so scores should stay near-perfect — any drop is a clear regression signal.

What it does

• Workflow (.github/workflows/lighthouse.yml): runs treosh/lighthouse-ci-action@v12 against the repo root served as a static site (no deploy wait). Least-privilege permissions: contents: read, concurrency cancels superseded runs, reports uploaded as workflow artifacts.
• Budgets (lighthouserc.json): hybrid gate — accessibility / SEO / best-practices hard-fail below 1.0; performance is warn-only (timing on shared CI runners fluctuates and would otherwise cause false failures).

Notes

• lighthouserc.json forces prefers-reduced-motion via chromeFlags. The page's entrance animation starts content at opacity: 0, which never advances in headless Lighthouse and yields NO_FCP; reduced motion makes content statically visible so the audit is deterministic. The gated categories are motion-independent.
• Verified locally through lhci (collect + assert): 3/3 runs at performance / accessibility / best-practices / SEO = 1.0, assert passes (exit 0).

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