docs: Security Best Practices Guide for Issue #47

[dawnraven-ai]

Bounty Claim for Issue #47

Comprehensive Security Best Practices Guide for PrivacyLayer users (~3,500 words).

Document Sections:

1. Wallet Security (Dedicated wallets, Hardware wallet, Backup/recovery, Freighter guidance)
2. Private Key Management (Key generation, Note management, Key rotation, MPC)
3. Transaction Security (Deposit/Withdrawal security, Monitoring, Replay attack prevention)
4. Smart Contract Interaction Security
5. Common Attack Prevention (Phishing, Social engineering, Replay, Front-running, Sybil)
6. User Operation Security Checklist
7. Emergency Response Procedures

Acceptance Criteria Met:

• ✅ Complete guide written
• ✅ 2000+ words (~3,500 words)
• ✅ Clear examples and practical guidance
• ✅ User-friendly language with do/don't sections
• ✅ Visual aids (checklists, structured formatting)

Closes #47