Add Lighthouse CI for browser-based performance checks

[Mandal-OS]

Goal

Add Lighthouse CI once a real web app or public website exists.

Why It Matters

The current audit CLI is useful for lightweight technical screening, but browser-based checks are needed for Core Web Vitals, render-blocking resources, accessibility, and SEO validation.

Initial Scope

• Add Lighthouse CI configuration.
• Run checks against local preview or deployed preview URL.
• Track LCP, CLS, accessibility, SEO, and best-practice scores.
• Document performance budgets and failure thresholds.

Definition of Done

• Lighthouse CI runs in GitHub Actions.
• Thresholds are documented and intentionally chosen.
• Reports are available from CI artifacts or logs.