Validate API configuration before making calls

[sfloess]

Part of #21: MEDIUM: No input validation before API calls

Scope

Validate that the API client is properly configured (API key set, endpoint URL valid, model selected) before attempting any API call.

What to do

• Add a validateConfiguration() method or pre-call check to each AI client
• Check for: missing/empty API key, missing/malformed endpoint URL, unset model name
• Throw a descriptive exception or show a user-facing notification if configuration is incomplete
• Consider a isConfigured() method for UI to disable "Send" button when not ready

Acceptance criteria

• Attempting an API call without a configured API key gives a clear error (not an HTTP 401 after the fact)
• Missing endpoint or model selection is caught before the HTTP call
• Users see actionable error messages directing them to the settings panel