Add Stellar network fee surge detection and user warning

[Mystery-CLI]

🔧 Title: Add Stellar network fee surge detection and user warning

📘 Description
During Stellar network congestion, the base fee can surge significantly. Users should be warned when fees are unusually high before submitting a payment, with the option to wait for fees to normalise.

✅ Acceptance Criteria

• Add fee surge detection: compare current fee to 7-day average
• Show a warning banner when the current fee is >5x the average
• Allow users to set a maximum fee they are willing to pay
• Add GET /api/stellar/network-status endpoint with fee surge indicator
• Add tests for surge detection logic

🔧 Context: backend/src/services/stellarNetwork.js; frontend/src/components/NetworkStatusBanner.jsx.

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