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/UsageScriptModal.tsx

@@ -253,7 +253,7 @@ const UsageScriptModal: React.FC<UsageScriptModalProps> = ({
                         timeout: parseInt(e.target.value),
                       })
                     }
-                    className="mt-1 w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded bg-white dark:bg-gray-800 text-gray-900 dark:text-gray-100"
+                    className="mt-1 w-full px-3 py-2 border border-border-default dark:border-border-default rounded bg-white dark:bg-gray-800 text-gray-900 dark:text-gray-100"
                   />
                 </label>
               </div>