feat(web): add React error boundaries and Next.js error.tsx fallbacks

Runtime errors in components now show a friendly "Something went wrong"
screen instead of a white page. Timeline and staffing panel are
individually wrapped. Route-level error.tsx handles server component errors.

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

apps/web/src/app/(app)/error.tsx

@@ -0,0 +1,60 @@
+"use client";
+
+import { useEffect } from "react";
+
+export default function AppError({
+  error,
+  reset,
+}: {
+  error: Error & { digest?: string };
+  reset: () => void;
+}) {
+  useEffect(() => {
+    console.error("[AppError]", error);
+  }, [error]);
+
+  return (
+    <div className="flex flex-col items-center justify-center min-h-[400px] p-8 text-center">
+      <div className="w-12 h-12 rounded-full bg-red-100 dark:bg-red-900/30 flex items-center justify-center mb-4">
+        <svg
+          className="w-6 h-6 text-red-600 dark:text-red-400"
+          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 1.732-3L13.732 4c-.77-1.333-2.694-1.333-3.464 0L3.34 16c-.77 1.333.192 3 1.732 3z"
+          />
+        </svg>
+      </div>
+      <h2 className="text-lg font-semibold text-gray-900 dark:text-gray-100 mb-2">
+        Something went wrong
+      </h2>
+      <p className="text-sm text-gray-500 dark:text-gray-400 mb-6 max-w-sm">
+        {error.message || "An unexpected error occurred. The team has been notified."}
+      </p>
+      {error.digest && (
+        <p className="text-xs text-gray-400 dark:text-gray-500 mb-4 font-mono">
+          Error ID: {error.digest}
+        </p>
+      )}
+      <div className="flex gap-3">
+        <button
+          onClick={reset}
+          className="px-4 py-2 text-sm font-medium text-white bg-indigo-600 hover:bg-indigo-700 rounded-lg transition-colors"
+        >
+          Try again
+        </button>
+        <button
+          onClick={() => (window.location.href = "/dashboard")}
+          className="px-4 py-2 text-sm font-medium text-gray-700 dark:text-gray-300 bg-gray-100 dark:bg-gray-800 hover:bg-gray-200 dark:hover:bg-gray-700 rounded-lg transition-colors"
+        >
+          Go to dashboard
+        </button>
+      </div>
+    </div>
+  );
+}

apps/web/src/app/(app)/not-found.tsx

@@ -0,0 +1,35 @@
+import Link from "next/link";
+
+export default function NotFound() {
+  return (
+    <div className="flex flex-col items-center justify-center min-h-[400px] p-8 text-center">
+      <div className="w-12 h-12 rounded-full bg-gray-100 dark:bg-gray-800 flex items-center justify-center mb-4">
+        <svg
+          className="w-6 h-6 text-gray-500 dark:text-gray-400"
+          fill="none"
+          stroke="currentColor"
+          viewBox="0 0 24 24"
+        >
+          <path
+            strokeLinecap="round"
+            strokeLinejoin="round"
+            strokeWidth={2}
+            d="M9.172 16.172a4 4 0 015.656 0M9 10h.01M15 10h.01M21 12a9 9 0 11-18 0 9 9 0 0118 0z"
+          />
+        </svg>
+      </div>
+      <h2 className="text-lg font-semibold text-gray-900 dark:text-gray-100 mb-2">
+        Page not found
+      </h2>
+      <p className="text-sm text-gray-500 dark:text-gray-400 mb-6 max-w-sm">
+        The page you are looking for does not exist or has been moved.
+      </p>
+      <Link
+        href="/dashboard"
+        className="px-4 py-2 text-sm font-medium text-white bg-indigo-600 hover:bg-indigo-700 rounded-lg transition-colors"
+      >
+        Go to dashboard
+      </Link>
+    </div>
+  );
+}

apps/web/src/app/(app)/staffing/page.tsx

@@ -1,5 +1,10 @@
 import { StaffingPanel } from "~/components/staffing/StaffingPanel.js";
+import { ErrorBoundary } from "~/components/ui/ErrorBoundary.js";
 
 export default function StaffingPage() {
-  return <StaffingPanel />;
+  return (
+    <ErrorBoundary>
+      <StaffingPanel />
+    </ErrorBoundary>
+  );
 }

apps/web/src/app/(app)/timeline/page.tsx

@@ -1,4 +1,5 @@
 import dynamic from "next/dynamic";
+import { ErrorBoundary } from "~/components/ui/ErrorBoundary.js";
 
 const TimelineView = dynamic(
   () => import("~/components/timeline/TimelineView.js").then((m) => m.TimelineView),
@@ -22,7 +23,9 @@ export default function TimelinePage() {
           <p className="app-page-subtitle mt-1">Interactive resource planning timeline</p>
         </div>
       </div>
-      <TimelineView />
+      <ErrorBoundary>
+        <TimelineView />
+      </ErrorBoundary>
     </div>
   );
 }