Add quick access to API key URLs via Tab key in settings

[brianlovin]

Summary

Added functionality to quickly open API key registration URLs directly from the settings panel. When a user selects a provider that doesn't have an API key configured, pressing Tab will open the appropriate provider's API key page.

Key Changes

• Added getSelectedProviderUrl() function to SettingsPanel that returns the API key URL for the currently selected provider if it lacks a configured key
• Integrated Tab key handler in the main app to detect when a provider without a key is selected and trigger the onOpenUrl callback
• Updated settings panel UI to display the API key URL and hint that Tab can be used to open it
• Changed hint text from generic "Add API Key" to show the actual provider URL (e.g., "console.anthropic.com (tab)")
• Updated hint color to use textTertiary for better visual hierarchy

Implementation Details

• The feature only activates when a provider item is selected that doesn't have a key configured
• Returns null if the selection is not a provider or if the provider already has a key configured
• Supports both Anthropic (console.anthropic.com) and OpenAI (platform.openai.com) providers
• Uses the existing onOpenUrl callback mechanism for opening URLs