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/CodexCommonConfigModal.tsx

@@ -53,7 +53,7 @@ export const CodexCommonConfigModal: React.FC<CodexCommonConfigModalProps> = ({
 
 # Add your common TOML configuration here`}
             rows={12}
-            className="w-full px-3 py-2 border border-gray-200 dark:border-gray-700 dark:bg-gray-800 dark:text-gray-100 rounded-lg text-sm font-mono focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20 focus:border-blue-500 dark:focus:border-blue-400 transition-colors resize-y"
+            className="w-full px-3 py-2 border border-border-default  dark:bg-gray-800 dark:text-gray-100 rounded-lg text-sm font-mono focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20 focus:border-border-active  transition-colors resize-y"
             autoComplete="off"
             autoCorrect="off"
             autoCapitalize="none"