fix(core): immutable state updates in store mutation functions

[drichar]

Description

This PR addresses the bug described in issue #219, where store mutation functions were inadvertently modifying state in unexpected ways. The fix ensures that the setActiveAccount, addWallet, and setAccounts functions create new object references for nested state properties, preventing unintended mutations of the original state.

Details

• Refactored setActiveAccount, addWallet, and setAccounts to create new object references for nested state properties (wallet state, accounts array, active account).
• Added unit tests to verify the fixes and ensure no unintended state mutations occur.
• These changes prevent unexpected behavior and potential loss of account information that could result from modifying shared object references.

Closes #219