fix: global wallet to use a mutex

[quantumshiro]

• Change the global wallet to use a Mutex to ensure thread safety
• Update references to the wallet to use the locked version
• Add mutex locking and unlocking to wallet methods that access the wallet

Fix balance check in create_transaction method

• Update the create_transaction method in the Wallet struct to correctly calculate the balance before checking if the amount exceeds the balance
• Print a message when the amount exceeds the balance

Update create_transaction method in tests

• Update the test_wallet_create_transaction function in the tests module to pass the Chain struct as an argument to the create_transaction method