Add support for hardware wallet signing (Ledger)

[Mystery-CLI]

🔧 Title: Add support for hardware wallet signing (Ledger)

📘 Description
Power users and high-value accounts should be able to sign Stellar transactions with a Ledger hardware wallet instead of storing their private key in the browser. The Stellar Ledger app supports this via the Stellar SDK.

✅ Acceptance Criteria

• Integrate @ledgerhq/hw-transport-webusb and the Stellar Ledger app
• Add a "Sign with Ledger" option in the payment confirmation dialog
• Handle Ledger connection errors gracefully
• Add a "Ledger Setup" guide in the security best practices documentation
• Add tests for the Ledger signing flow (mock transport)

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

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