Add support for Stellar federation addresses

[Mystery-CLI]

🔧 Title: Add support for Stellar federation addresses

📘 Description
Stellar federation allows users to use human-readable addresses like alice*futureremit.app instead of raw public keys. This dramatically improves UX for sending payments.

✅ Acceptance Criteria

• Add federation address resolution in the payment form (detect * in the recipient field)
• Add GET /api/stellar/federation?address=alice*futureremit.app endpoint
• Implement a basic federation server at /.well-known/stellar.toml
• Allow users to claim a federation address in account settings
• Add tests for federation resolution

🔧 Context: frontend/src/App.jsx recipient field; backend/src/routes/stellar.js.

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