feat: add send_public_transaction for arbitrary program transactions

[adklempner]

Purpose

Expose send_public_transaction in the wallet module so logos-core modules can submit arbitrary program transactions without per-program FFI changes.

Related: logos-blockchain/logos-execution-zone#422
Depends on logos-blockchain/logos-execution-zone#429

Changes

Adds a single Q_INVOKABLE method send_public_transaction(const QString& tx_json) that:

• Accepts a JSON object: {"program_id":"hex", "accounts":["hex",...], "instruction":"hex", "signer_account":"hex"}
• Parses and validates all hex fields
• Calls the underlying wallet_ffi_send_public_transaction FFI function
• Returns the transaction result as JSON

Checklist

• Complete PR description
• Implement the core functionality
• Add/update tests