Add check button for API keys and LLM endpoints in admin dashboard

[wroersma]

Implement a check button next to the input fields for API keys and LLM endpoints in the admin dashboard. This will allow users to verify the validity of the entered keys/endpoints before saving the configuration.

Acceptance criteria:

• A check button is displayed next to each API key and LLM endpoint input field in the admin dashboard.
• Clicking the check button triggers a validation process.
• The validation process verifies the validity of the entered key/endpoint.
• The UI provides feedback to the user indicating whether the key/endpoint is valid or invalid.