fix(layout): add React error boundary to catch render crashes

Previously a JS rendering error produced a blank white screen with no
feedback. PageErrorBoundary now catches the error, shows the error
message + stack trace, and offers a reload button. This will surface
the exact crash message for the inaccessible test page.

frontend/src/components/Layout.tsx

@@ -1,8 +1,64 @@
 import { Outlet } from "react-router-dom";
-import { LogOut } from "lucide-react";
+import { LogOut, AlertTriangle, RefreshCw } from "lucide-react";
 import { useAuth } from "../context/AuthContext";
 import Sidebar from "./Sidebar";
 import NotificationBell from "./NotificationBell";
+import React from "react";
+
+/* ── Error Boundary ──────────────────────────────────────────────────
+   Catches any unhandled rendering error and shows a recoverable error
+   screen instead of a blank white page.
+   ─────────────────────────────────────────────────────────────────── */
+interface EBState { hasError: boolean; error: Error | null }
+
+class PageErrorBoundary extends React.Component<{ children: React.ReactNode }, EBState> {
+  constructor(props: { children: React.ReactNode }) {
+    super(props);
+    this.state = { hasError: false, error: null };
+  }
+
+  static getDerivedStateFromError(error: Error): EBState {
+    return { hasError: true, error };
+  }
+
+  componentDidCatch(error: Error, info: React.ErrorInfo) {
+    console.error("[PageErrorBoundary]", error, info.componentStack);
+  }
+
+  render() {
+    if (this.state.hasError) {
+      return (
+        <div className="flex h-full flex-col items-center justify-center gap-4 p-8 text-center">
+          <AlertTriangle className="h-12 w-12 text-red-400" />
+          <div>
+            <h2 className="text-xl font-semibold text-white">Something went wrong</h2>
+            <p className="mt-1 text-sm text-gray-400">
+              {this.state.error?.message ?? "An unexpected error occurred while rendering this page."}
+            </p>
+            {this.state.error?.stack && (
+              <pre className="mt-3 max-h-40 overflow-y-auto rounded-lg border border-gray-800 bg-gray-900 p-3 text-left text-xs text-gray-500">
+                {this.state.error.stack}
+              </pre>
+            )}
+          </div>
+          <button
+            onClick={() => {
+              this.setState({ hasError: false, error: null });
+              window.location.reload();
+            }}
+            className="flex items-center gap-2 rounded-lg border border-gray-700 px-4 py-2 text-sm text-gray-300 hover:bg-gray-800"
+          >
+            <RefreshCw className="h-4 w-4" />
+            Reload page
+          </button>
+        </div>
+      );
+    }
+    return this.props.children;
+  }
+}
+
+/* ── Layout ─────────────────────────────────────────────────────────── */
 
 export default function Layout() {
   const { user, logout } = useAuth();
@@ -25,9 +81,11 @@ export default function Layout() {
           </button>
         </header>
 
-        {/* Main content */}
+        {/* Main content wrapped in error boundary */}
         <main className="flex-1 overflow-y-auto p-6">
-          <Outlet />
+          <PageErrorBoundary>
+            <Outlet />
+          </PageErrorBoundary>
         </main>
       </div>
     </div>