Add an installable web app manifest for the wallet

[collinsadi]

frontend/public/ ships no manifest.webmanifest, so the wallet cannot be installed to a home screen and has no app identity, despite holding local-only spend-critical data users want quick access to.

Add a web app manifest with icons and installability.

Acceptance criteria:

• The app passes installability checks in Chromium and Safari.
• Icons and theme colors match the branding.
• Install behavior is documented, including its interaction with the existing service-worker caching policy.