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/UpdateBadge.tsx

@@ -21,7 +21,7 @@ export function UpdateBadge({ className = "", onClick }: UpdateBadgeProps) {
       className={`
         flex items-center gap-1.5 px-2.5 py-1
         bg-white dark:bg-gray-800
-        border border-gray-200 dark:border-gray-700
+        border border-border-default 
         rounded-lg text-xs
         shadow-sm
         transition-all duration-200