OpenRouter api key support

there is no direct way to use an OpenRouter API key for authentication.

Why this is needed: • OpenRouter allows routing AI API requests through multiple models/providers with a single API key. • Supporting OpenRouter would make it easier for users to switch between AI models without changing much in their setup. • It provides additional availability in regions where certain API providers are restricted.

Proposed Solution: • Add configuration option for OPENROUTER_API_KEY (environment variable). • Allow selecting OpenRouter as a provider in settings/config.