docs: Add wallet migration guide (Part 1 → Part 2)

[dominiccreates]

This pr closes #400
This PR introduces the Wallet Migration Guide (WALLET_MIGRATION_GUIDE.md) to document the transition for users from legacy connected external wallets (Part 1 - e.g., Freighter, MetaMask, Phantom) to the auto-generated embedded wallet system (Part 2 - invisible "smart wallets" generated automatically on signup).

This fulfills the documentation requirements without altering any application logic or source code files.

🚀 Key Benefits Documented

• Improved Security: Explains how the auto-generated system secures keys without browser storage.
• Simplified Onboarding: Highlights the frictionless Web2 flow (no browser extension installs required).
• Cross-Chain Compatibility: Stellar, Ethereum, and Solana support.

📝 What is Covered in the Guide?

1. Overview & Rationale: The difference between connected wallets (Part 1) and auto-generated wallets (Part 2).
2. Prerequisites Checklist: Requirements before beginning migration.
3. Step-by-Step Migration: Detailed UI and transaction flow instructions.
4. Post-Migration Verification: Steps to verify that NFTs, tokens, and assets are successfully linked.
5. Frequently Asked Questions (FAQ): Reassuring details regarding legacy keys, revert capability, and network fees.

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