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/mcp/McpFormModal.tsx

@@ -626,7 +626,7 @@ const McpFormModal: React.FC<McpFormModalProps> = ({
                 <input
                   id={syncCheckboxId}
                   type="checkbox"
-                  className="h-4 w-4 rounded border-gray-300 text-emerald-600 focus:ring-emerald-500 dark:border-gray-600 dark:bg-gray-800"
+                  className="h-4 w-4 rounded border-border-default text-emerald-600 focus:ring-emerald-500  dark:bg-gray-800"
                   checked={syncOtherSide}
                   onChange={(event) => setSyncOtherSide(event.target.checked)}
                 />