Add offline transaction signing support

[Just-Bamford]

Problem

Users can't sign transactions offline (air-gapped scenarios). All signing requires live wallet connection.

Solution

Implement \signTransactionOffline(xdr, privateKey)\ in \src/wallet/index.ts. Allows manual key-based signing without wallet.

Acceptance Criteria

• Function added to \src/wallet/index.ts\
• Accepts XDR and private key
• Returns signed XDR
• Tests cover signing

Note for contributors

Use Stellar SDK signing. Reference keypair handling. Add tests to \src/tests/wallet.test.ts.