frontend: Lighthouse perf & accessibility baseline for cauweb

[ayushkrtiwari]

Goal

Establish measurable performance and accessibility baselines and fix top issues.

Tasks

• Add GitHub Action to run Lighthouse (CI) on staging deploys and post a report artifact.
• Fix top 10 performance regressions (unused JS, large bundle, render-blocking CSS).
• Fix top 10 accessibility items flagged by Lighthouse.
• Document target thresholds (e.g., LCP < 2.5s, Accessibility > 90).

Acceptance criteria

• Lighthouse report attached to PRs/CI with thresholds enforced on staging.