refactor: drop WALLET_PROVIDER_USER_URL and derive the URL default from the port

[gabitoesmiapodo]

Summary

Closes #2

WALLET_PROVIDER_USER_URL had no reader and named a human-facing wallet page this service does not serve. WALLET_PROVIDER_URL fell back to a fixed http://localhost:3010, so moving the bind port left the advertised URL pointing at a port nothing listens on.

Changes

• WALLET_PROVIDER_USER_URL is gone, with userUrl on the provider descriptor and the connect result
• The WALLET_PROVIDER_URL fallback is now http://localhost: plus the port actually bound
• An explicit WALLET_PROVIDER_URL is still used verbatim, port or no port

Deviations

• .env.example carries no explanatory comment on WALLET_PROVIDER_URL, dropped by decision
• README has no provider config table to update; the WALLET_PROVIDER_* row in architecture.md stays accurate

Acceptance criteria

• WALLET_PROVIDER_URL defaults to http://localhost:${WALLET_SERVICE_PORT}
• An explicit WALLET_PROVIDER_URL overrides the default, with or without a port
• WALLET_PROVIDER_USER_URL is removed from src/config.ts
• userUrl is removed from buildProvider and from the connect result
• .env.example drops WALLET_PROVIDER_USER_URL and comments WALLET_PROVIDER_URL as a deployment override
• The README config table reflects both changes

Test plan

Automated tests

1. Run pnpm install
2. Run pnpm test, expect 91 passing and 0 failing
3. Run pnpm run typecheck, pnpm knip and pnpm run build, expect no output from any

Manual verification

1. Check out this branch, cp .env.example .env, fill CANTON_BACKEND_TOKEN
2. Delete the WALLET_PROVIDER_URL line from .env and set WALLET_SERVICE_PORT=4000
3. Run pnpm run dev
4. Run curl -s localhost:4000/rpc -H 'content-type: application/json' -d '{"jsonrpc":"2.0","id":1,"method":"status"}'
5. Expect provider.url to read http://localhost:4000, and no userUrl in the response

Breaking changes

userUrl no longer appears in the status provider descriptor or the connect result. It is optional in the vendored OpenRPC spec, so no consumer contract required it. No migration.

Checklist

• Self-reviewed my own diff
• Tests added or updated
• Docs updated (if applicable)
• No unrelated changes bundled in

Screenshots

None.