refactor: complete border unification across all components

- Add border styles to JsonEditor (CodeMirror) with theme-responsive colors
- Update all dialog header/footer dividers to use border-border-default
- Replace remaining border-border instances in settings components
- Ensure all borders (including separators and container borders) use unified design system
- All borders now consistently use CSS variables and respond to light/dark themes

src/components/settings/AboutSection.tsx

@@ -140,7 +140,7 @@ export function AboutSection({ isPortable }: AboutSectionProps) {
         </p>
       </header>
 
-      <div className="space-y-4 rounded-lg border border-border p-4">
+      <div className="space-y-4 rounded-lg border border-border-default p-4">
         <div className="flex flex-col gap-4 sm:flex-row sm:items-center sm:justify-between">
           <div className="space-y-1">
             <p className="text-sm font-medium text-foreground">CC Switch</p>