Add provider profiles and connection controls to Settings #37

Goal

Let users configure and select AI providers without exposing implementation details.

Depends on

• Define a provider-neutral AI client and capability contract #35 and Move AI execution and credentials into a secure Electron main-process service #36.

Scope

Add provider settings for:

• provider selection;
• API key when required;
• base URL when supported;
• model selection;
• Test Connection;
• Fetch Models;
• Set Default.

Include clear copy that ChatGPT/Claude subscriptions do not include API access, and that cloud-provider requests send selected note context off-device.

Acceptance criteria

• Validation and capability-dependent fields are clear.
• Keys are submitted to the main process immediately and are not retained in renderer state longer than needed.
• Connection errors are actionable and do not include secrets.
• Keyboard navigation and narrow-window layouts work.
• No automatic provider switching or fallback-chain UI is introduced.