[codex] Preserve and document area PWA

[fik14]

What changed

• add preservation-focused project documentation and live-app guidance
• replace the generic PWA manifest name
• use a repository-specific service-worker cache and precache icons
• render the user-controlled currency label with textContent instead of innerHTML

Why

This keeps the legacy utility easy to understand and install while addressing the small DOM-injection risk found during the audit.

Impact

The app remains dependency-free and its existing calculation behavior and public URL are unchanged.

Validation

• git diff --check
• manifest parsed with python3 -m json.tool
• local HTTP smoke checks returned 200 for the app, manifest, and icon assets