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/ui/dialog.tsx

@@ -78,7 +78,7 @@ const DialogHeader = ({
 }: React.HTMLAttributes<HTMLDivElement>) => (
   <div
     className={cn(
-      "flex flex-col space-y-1.5 text-center sm:text-left px-6 py-5 border-b border-border bg-muted/20 flex-shrink-0",
+      "flex flex-col space-y-1.5 text-center sm:text-left px-6 py-5 border-b border-border-default bg-muted/20 flex-shrink-0",
       className,
     )}
     {...props}
@@ -92,7 +92,7 @@ const DialogFooter = ({
 }: React.HTMLAttributes<HTMLDivElement>) => (
   <div
     className={cn(
-      "flex flex-col-reverse gap-2 sm:flex-row sm:justify-end sm:items-center px-6 py-5 border-t border-border bg-muted/20 flex-shrink-0",
+      "flex flex-col-reverse gap-2 sm:flex-row sm:justify-end sm:items-center px-6 py-5 border-t border-border-default bg-muted/20 flex-shrink-0",
       className,
     )}
     {...props}