feat: add React ErrorBoundary with retry and reload UI

Add class-based ErrorBoundary component that catches rendering errors
and shows a user-friendly fallback with retry/reload buttons. Wrap in
root layout to protect against full-page crashes.

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

frontend/src/app/layout.tsx

@@ -3,6 +3,7 @@ import { Inter, Noto_Sans_KR } from 'next/font/google';
 import './globals.css';
 import { ThemeProvider } from '@/components/providers/theme-provider';
 import { Toaster } from '@/components/ui/sonner';
+import { ErrorBoundary } from '@/components/error-boundary';
 
 const inter = Inter({
   subsets: ['latin'],
@@ -34,7 +35,9 @@ export default function RootLayout({
           enableSystem
           disableTransitionOnChange
         >
-          {children}
+          <ErrorBoundary>
+            {children}
+          </ErrorBoundary>
           <Toaster />
         </ThemeProvider>
       </body>

frontend/src/components/error-boundary.tsx

@@ -0,0 +1,73 @@
+'use client';
+
+import React from 'react';
+import { Button } from '@/components/ui/button';
+
+interface ErrorBoundaryProps {
+  children: React.ReactNode;
+}
+
+interface ErrorBoundaryState {
+  hasError: boolean;
+}
+
+export class ErrorBoundary extends React.Component<ErrorBoundaryProps, ErrorBoundaryState> {
+  constructor(props: ErrorBoundaryProps) {
+    super(props);
+    this.state = { hasError: false };
+  }
+
+  static getDerivedStateFromError(): ErrorBoundaryState {
+    return { hasError: true };
+  }
+
+  componentDidCatch(error: Error, errorInfo: React.ErrorInfo) {
+    console.error('ErrorBoundary caught an error:', error, errorInfo);
+  }
+
+  handleRetry = () => {
+    this.setState({ hasError: false });
+  };
+
+  render() {
+    if (this.state.hasError) {
+      return (
+        <div className="flex min-h-screen items-center justify-center bg-background">
+          <div className="text-center space-y-4 p-8">
+            <div className="inline-flex items-center justify-center w-16 h-16 rounded-full bg-destructive/10 mb-2">
+              <svg
+                className="w-8 h-8 text-destructive"
+                fill="none"
+                stroke="currentColor"
+                viewBox="0 0 24 24"
+              >
+                <path
+                  strokeLinecap="round"
+                  strokeLinejoin="round"
+                  strokeWidth={2}
+                  d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-2.5L13.732 4c-.77-.833-1.964-.833-2.732 0L4.082 16.5c-.77.833.192 2.5 1.732 2.5z"
+                />
+              </svg>
+            </div>
+            <h2 className="text-xl font-semibold text-foreground">
+              문제가 발생했습니다
+            </h2>
+            <p className="text-muted-foreground">
+              페이지를 새로고침 해주세요.
+            </p>
+            <div className="flex gap-3 justify-center">
+              <Button onClick={this.handleRetry} variant="outline">
+                다시 시도
+              </Button>
+              <Button onClick={() => window.location.reload()}>
+                페이지 새로고침
+              </Button>
+            </div>
+          </div>
+        </div>
+      );
+    }
+
+    return this.props.children;
+  }
+}