Mock bank demo: add reset config button

[yaront1111]

Context

The mock bank demo stores config in localStorage under cordum-mock-bank-config. If the API key/base URL changes, the demo can get stuck in 401/404 until the user clears storage.

Task

Add a small UI control to reset the demo config to defaults.

Acceptance criteria

• Add a button/link in demo/mock-bank/index.html (header/footer is fine).
• On click, clear localStorage key cordum-mock-bank-config and reload the page.
• Styling matches existing demo (small, unobtrusive).