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/ui/button.tsx

@@ -16,7 +16,7 @@ const buttonVariants = cva(
           "bg-red-500 text-white hover:bg-red-600 dark:bg-red-600 dark:hover:bg-red-700",
         // 轮廓按钮
         outline:
-          "border border-gray-200 bg-background hover:bg-gray-100 dark:border-gray-700 dark:hover:bg-gray-800",
+          "border border-border-default bg-background hover:bg-gray-100 hover:border-border-hover dark:hover:bg-gray-800",
         // 次按钮：灰色（对应旧版 secondary）
         secondary:
           "text-gray-500 hover:bg-gray-100 dark:text-gray-400 dark:hover:bg-gray-800 dark:hover:text-gray-200",