Add browser demo for FDE presentations

[echowang1]

Summary

• add a static browser demo for presenting Gauntlet payment preflight scenarios
• include scenario selection, editable payment request controls, decision output, redacted receipt preview, and FDE talk track
• add a field demo playbook and README entry

Verification

• node --check demo/app.js
• npm run typecheck
• npm test
• npm run build
• Chrome headless desktop and mobile screenshots