Issue 073: Add pay_per_use_to recipient flow with whitelist validation messaging

[orunganiekan]

Category: Frontend
Complexity: 200 points
Labels: frontend, wave-200

Context

Contract supports pay_per_use_to with recipient whitelist checks; frontend primarily wraps pay_per_use.

Problem

Users cannot route one-shot payments to an alternate recipient when merchants support it.

Goal

Optional recipient field with validation and clear MerchantNotWhitelisted errors.

Scope

Includes: form + stellar builder + tests.
Does NOT include: recipient marketplace.

Implementation Guidelines

• Files: PayPerUseForm.tsx, stellar.ts, errors mapper.

Acceptance Criteria

• Recipient optional
• Builds pay_per_use_to when set
• Client validation for address
• Tests included

Validation

Vitest.

PR Expectations

The PR should include:

• Recipient flow + tests

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