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/AppSwitcher.tsx

@@ -13,7 +13,7 @@ export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
   };
 
   return (
-    <div className="inline-flex bg-gray-100 dark:bg-gray-800 rounded-lg p-1 gap-1 border border-transparent dark:border-gray-700">
+    <div className="inline-flex bg-gray-100 dark:bg-gray-800 rounded-lg p-1 gap-1 border border-transparent ">
       <button
         type="button"
         onClick={() => handleSwitch("claude")}