fix(web): harden typed SPA chat lifecycle

Co-Authored-By: Claude Haiku 4.5 <[email protected]>

apps/web/src/spa/pages/chat-error-boundary.tsx

@@ -0,0 +1,22 @@
+import type { ReactElement } from 'react';
+import { useRouteError } from 'react-router-dom';
+
+/**
+ * `/chat` renders live, server-driven state (streamed text, tool calls,
+ * manifests) that can carry malformed payloads no compile-time contract can
+ * fully rule out at every dereference site. This is the last line of
+ * defense: if something still throws during render, show a recoverable
+ * alert instead of leaving the user on a blank/white screen.
+ */
+export function ChatRouteErrorBoundary(): ReactElement {
+  useRouteError();
+
+  return (
+    <div role="alert" className="flex min-h-screen flex-col items-center justify-center gap-3 p-8">
+      <p className="text-sm font-medium">Something went wrong loading chat.</p>
+      <a href="/chat" className="text-sm underline">
+        Reload chat
+      </a>
+    </div>
+  );
+}