Pay pages: one link that collects private payments

Day two of the network pivot. A pay page is a serverless hosted surface —
name, emoji, presets and address travel in the URL fragment (the /pay/name
path segment is cosmetic), so lumen.app/pay/you works with no backend and
no gas. Payments to it settle as private transfers: no public sender or
amount, and no payer can see any other.

- lib/lumen/paypage.ts: fragment codec (standing page + exact-amount
  request lock), slugify, device-local page settings; 6 vitest cases
- /pay/[[...slug]]: the payer surface — USD presets priced live, guard
  review with the payer's own ledger, wallet connect, Ready onboarding for
  wallet-less visitors, self-pay detection, receipt on success
- Get paid sheet: configure the standing page or mint a request link
  ("invoice a client"), reachable from Receive and the menu

172 TS tests green; build ships /pay as a dynamic route.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>