0. Add `lib/utils/validation.ts` Stellar address validation

[DeFiVC]

What: Add Stellar address validation utilities.

Why: Stellar addresses need validation (56 characters, starts with G, valid checksum). A utility module centralizes this.

Scope: Add validators for: isValidStellarAddress, isValidEd25519PublicKey, isValidContractId. Use Stellar SDK where possible.

Acceptance criteria:

• Address validation works
• Contract ID validation works
• Error messages are clear
• Edge cases are handled

Technical context: src/lib/utils/validation.ts — validation utils. src/lib/stellar/wallet.ts — wallet module.

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