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)/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>
+  );
+}