Add a "Request Payment" feature with shareable links

[Mystery-CLI]

🔧 Title: Add a "Request Payment" feature with shareable links

📘 Description
Users can only send payments, not request them. A "Request Payment" feature that generates a shareable link or QR code pre-filled with the recipient address and amount would be valuable for merchants and individuals.

✅ Acceptance Criteria

• Add a "Request Payment" button that opens a form for amount and memo
• Generate a web+stellar:pay?destination=...&amount=...&memo=... URI
• Display the URI as a QR code and a copyable link
• Add GET /api/stellar/payment-request/:id endpoint for persistent request links
• Add tests for URI generation and QR code display

🔧 Context: frontend/src/components/QRCodeModal.jsx; backend/src/routes/stellar.js.

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