refactor: implement unified border design system

- Define custom border utilities in @layer utilities for consistent theming
- Add border-default (1px gray), border-active (2px primary), border-hover (40% primary), and border-dragging (60% primary) classes
- Update all UI components (Input, Select, TextArea, Button, Dialog, Dropdown) to use unified border classes
- Replace hardcoded border colors (gray-200/300/600/700) with theme-responsive border-border-default
- Update provider cards, MCP components, settings, and forms with new border system
- Remove dark mode border overrides to simplify CSS and improve maintainability
- Ensure all borders automatically adapt to light/dark themes via CSS variables

src/components/providers/forms/CodexQuickWizardModal.tsx

@@ -239,7 +239,7 @@ export const CodexQuickWizardModal: React.FC<CodexQuickWizardModalProps> = ({
 
           {/* Preview */}
           {(templateApiKey || templateProviderName || templateBaseUrl) && (
-            <div className="space-y-2 border-t border-gray-200 pt-4 dark:border-gray-700">
+            <div className="space-y-2 border-t border-border-default pt-4 ">
               <h3 className="text-sm font-medium text-gray-900 dark:text-gray-100">
                 {t("codexConfig.configPreview")}
               </h3>