docs: add comprehensive end-user documentation

[wocaoac-cpu]

Summary

• Add docs/USER_GUIDE.md with user-friendly documentation for non-technical users
• Covers the complete user journey: wallet setup, deposits, withdrawals, note management, security, and privacy
• Written in clear, non-technical language with prominent security warnings about note backup
• Includes comprehensive FAQ and troubleshooting sections

Document Structure

1. Introduction -- What is PrivacyLayer, how it works (simple explanation), privacy guarantees, limitations
2. Getting Started -- Freighter wallet setup, funding, accessing the dApp
3. Making a Deposit -- Step-by-step with denomination selection, note generation, backup warning, confirmation
4. Making a Withdrawal -- Step-by-step with note entry, recipient address, proof generation, submission
5. Note Management -- Note format explanation, secure backup methods, recovery (impossible), multiple notes
6. Security Best Practices -- Note protection, different recipient addresses, timing, avoiding patterns
7. Privacy Considerations -- Anonymity set size, timing attacks, amount correlation, network privacy (VPN/Tor)
8. Troubleshooting -- Common errors table, stuck transactions, proof generation failures
9. FAQ -- General, privacy, technical, cost, and safety questions

Acceptance Criteria

• Complete USER_GUIDE.md file
• Over 1500 words (document is ~3400 words)
• Security warnings prominently displayed throughout
• FAQ section with 15+ questions across 5 categories
• Non-technical language throughout
• Reviewed for clarity and completeness

Closes #20