Replace hardcoded Tailwind color classes with design system CSS variables
to improve text visibility in dark mode:
- text-gray-900 dark:text-gray-100 → text-foreground
- text-gray-500/600 dark:text-gray-400 → text-muted-foreground
- bg-white dark:bg-gray-800 → bg-background
- Migrate all form components from ProviderForm/ to providers/forms/
- Create shared components to eliminate code duplication:
* ApiKeySection: unified API key input with "Get API Key" link
* EndpointField: unified endpoint URL input with manage button
- Refactor ClaudeFormFields (-31% lines) and CodexFormFields (-33% lines)
- Update all import paths to use new locations
- Reduce code duplication from ~12% to ~7%
This change improves maintainability and makes the codebase more DRY.