Add PaymentDestination interface for lightweight server destinations

[badjer]

Summary

• Adds PaymentDestination interface to @atxp/common - minimal interface for server destinations
• Updates Account type to extend PaymentDestination (backwards compatible)
• Adds AccountIdDestination class - lightweight destination using just an accountId
• Updates server ATXPConfig.destination type from Account to PaymentDestination

Motivation

Sprites and other services that receive payments don't need a full connection_token to operate. They only need:

• getAccountId() - to identify where payments go
• getSources() - to provide payment options (public endpoint, no auth)

This change allows servers to use:

import { AccountIdDestination } from '@atxp/common';
const destination = new AccountIdDestination('abc123');

Instead of requiring a full connection string:

import { ATXPAccount } from '@atxp/common';
const destination = new ATXPAccount(process.env.ATXP_CONNECTION_STRING!);

Changes

New interface (types.ts):

export interface PaymentDestination {
  getAccountId: () => Promise<AccountId>;
  getSources: () => Promise<Source[]>;
}

Updated Account (types.ts):

export type Account = PaymentDestination & {
  paymentMakers: PaymentMaker[];
}

New class (accountIdDestination.ts):

• Takes accountId string (qualified atxp:abc123 or unqualified abc123)
• Calls public /account/{id}/sources endpoint (no auth needed)
• No connection_token required

Test plan

• All existing tests pass
• Manual test with sprite using AccountIdDestination

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