Add support for recurring payments via the streaming UI

[Mystery-CLI]

🔧 Title: Add support for recurring payments via the streaming UI

📘 Description
StreamPayment.jsx and the streaming service implement payment streams, but the UI may not clearly communicate the concept of recurring payments to non-technical users. The UX should be as simple as setting up a standing order at a bank.

✅ Acceptance Criteria

• Redesign the stream creation UI with plain-language labels ("Send X XLM every Y days")
• Add a calendar-style end date picker
• Show a summary: "Total to be sent: X XLM over Y days"
• Add a "Pause" and "Cancel" button to active streams
• Add tests for the stream creation form

🔧 Context: frontend/src/components/StreamPayment.jsx.

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