feat: add authorizationRelay handler

[figtracer]

Description:

adds Handler.authorizationRelay() — a handler that serves an /authorize page for cross-domain access key authorization.

problem

every mpp-powered app currently needs its own passkey wallet per domain (webauthn rpId limitation). users have to create a new wallet and fund it separately for each app. this kills adoption.

solution

a relay page that lets third-party apps authorize access keys against the user's existing tempo wallet via popup + postMessage.

flow:

1. third-party app (e.g. varzim.figtracer.com) opens popup to tempo.xyz/authorize?keyAddress=0x...&hash=0x...
2. relay page shows "authorize access key?" with key address, expiry, requesting origin
3. user approves with their tempo.xyz passkey
4. signed credential is sent back to the opener via postMessage
5. third-party app uses the access key to sign mpp payments from the user's main wallet

same pattern as tempo wallet login in the CLI — but for the browser.

usage:

const relay = Handler.authorizationRelay({
  rp: { id: 'tempo.xyz' },
})

companion PR: wevm/wagmi — crossDomainAccessKey connector wevm/wagmi#5027