feat: add frontend source code

Add search-zen-50 React SPA source code to frontend/ directory.
Build artifacts (dist, node_modules, lock files) are gitignored.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/ui/radio-group.tsx

@@ -0,0 +1,36 @@
+import * as React from "react";
+import * as RadioGroupPrimitive from "@radix-ui/react-radio-group";
+import { Circle } from "lucide-react";
+
+import { cn } from "@/lib/utils";
+
+const RadioGroup = React.forwardRef<
+  React.ElementRef<typeof RadioGroupPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof RadioGroupPrimitive.Root>
+>(({ className, ...props }, ref) => {
+  return <RadioGroupPrimitive.Root className={cn("grid gap-2", className)} {...props} ref={ref} />;
+});
+RadioGroup.displayName = RadioGroupPrimitive.Root.displayName;
+
+const RadioGroupItem = React.forwardRef<
+  React.ElementRef<typeof RadioGroupPrimitive.Item>,
+  React.ComponentPropsWithoutRef<typeof RadioGroupPrimitive.Item>
+>(({ className, ...props }, ref) => {
+  return (
+    <RadioGroupPrimitive.Item
+      ref={ref}
+      className={cn(
+        "aspect-square h-4 w-4 rounded-full border border-primary text-primary ring-offset-background focus:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50",
+        className,
+      )}
+      {...props}
+    >
+      <RadioGroupPrimitive.Indicator className="flex items-center justify-center">
+        <Circle className="h-2.5 w-2.5 fill-current text-current" />
+      </RadioGroupPrimitive.Indicator>
+    </RadioGroupPrimitive.Item>
+  );
+});
+RadioGroupItem.displayName = RadioGroupPrimitive.Item.displayName;
+
+export { RadioGroup, RadioGroupItem };