Add first-run configuration screen

[bennekrouf]

What changed

• Added a first-run configuration screen to the application.
• Updated CSS to style the new first-run screen.
• Modified the main application logic to show the first-run screen on initial launch if the application is not yet configured.
• Added a new service flow::without_model to handle steps when no model is configured.
• Updated the llm service to include a new ProviderKind::Off for no model configuration.
• Added a new screen component FirstRun for the first-run configuration.

Why

The first-run configuration screen is necessary to ask the user what, if anything, writes the commit messages. This is a critical choice that affects the entire workflow of the application. The screen preselects based on the detected environment and allows the user to choose between running a local model, using a hosted API, or running without AI. This ensures a smooth onboarding experience and prevents the user from getting stuck in a configuration that doesn't work for them.

How to test

1. Launch the application for the first time.
2. Ensure the first-run configuration screen appears.
3. Test each option (Run a model on this machine, Use a hosted API, No AI) to see the corresponding UI changes and messages.
4. Complete the configuration and proceed to the welcome screen.
5. Verify that the application settings are saved and the chosen configuration is applied.