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/CommonConfigEditor.tsx

@@ -60,7 +60,7 @@ export function CommonConfigEditor({
                 id="useCommonConfig"
                 checked={useCommonConfig}
                 onChange={(e) => onCommonConfigToggle(e.target.checked)}
-                className="w-4 h-4 text-blue-500 bg-white dark:bg-gray-800 border-gray-300 dark:border-gray-600 rounded focus:ring-blue-500 dark:focus:ring-blue-400 focus:ring-2"
+                className="w-4 h-4 text-blue-500 bg-white dark:bg-gray-800 border-border-default  rounded focus:ring-blue-500 dark:focus:ring-blue-400 focus:ring-2"
               />
               <span>
                 {t("claudeConfig.writeCommonConfig", {