feat: add error boundary to prevent white-screen crashes

Wraps the entire app in an ErrorBoundary that shows a friendly
error message with reload button instead of crashing to white.

frontend/src/components/ErrorBoundary.tsx

@@ -0,0 +1,44 @@
+import { Component, type ErrorInfo, type ReactNode } from 'react';
+
+interface Props {
+  children: ReactNode;
+}
+
+interface State {
+  hasError: boolean;
+  error: Error | null;
+}
+
+export class ErrorBoundary extends Component<Props, State> {
+  state: State = { hasError: false, error: null };
+
+  static getDerivedStateFromError(error: Error): State {
+    return { hasError: true, error };
+  }
+
+  componentDidCatch(error: Error, errorInfo: ErrorInfo) {
+    console.error('ErrorBoundary caught:', error, errorInfo);
+  }
+
+  render() {
+    if (this.state.hasError) {
+      return (
+        <div className="flex items-center justify-center min-h-screen bg-background">
+          <div className="text-center p-8 max-w-md">
+            <h1 className="text-xl font-bold mb-2 text-foreground">Something went wrong</h1>
+            <p className="text-muted-foreground mb-4 text-sm">
+              {this.state.error?.message ?? 'An unexpected error occurred.'}
+            </p>
+            <button
+              onClick={() => window.location.reload()}
+              className="px-4 py-2 bg-primary text-primary-foreground rounded-md text-sm font-medium hover:bg-primary/90 transition-colors"
+            >
+              Reload Page
+            </button>
+          </div>
+        </div>
+      );
+    }
+    return this.props.children;
+  }
+}