fix(ui): contain routed-page render errors with a boundary

A render-time throw on a routed page (e.g. the New Agent page at
/agents/new?adapterType=claude_local) had no boundary, so it unmounted
the whole SPA and left a blank page. Because the custom router updates
location without remounting, the back button changed the URL but the
tree stayed dead until a hard refresh.

Wrap the routed <Outlet/> in a RouteErrorBoundary that renders a
recoverable error card, keeps the sidebar/nav mounted, and resets its
error state whenever the route (pathname+search) changes so navigating
away — back button included — recovers without a hard refresh.

ui/src/components/Layout.tsx

@@ -17,6 +17,7 @@ import { MobileBottomNav } from "./MobileBottomNav";
 import { WorktreeBanner } from "./WorktreeBanner";
 import { DevRestartBanner } from "./DevRestartBanner";
 import { StandaloneBrowserControls } from "./StandaloneBrowserControls";
+import { RouteErrorBoundary } from "./RouteErrorBoundary";
 import { SidebarShell } from "./SidebarShell";
 import { SecondarySidebar } from "./SecondarySidebar";
 import { SidebarAccountMenu } from "./SidebarAccountMenu";
@@ -566,7 +567,9 @@ export function Layout() {
                   requestedPrefix={companyPrefix ?? selectedCompany?.issuePrefix}
                 />
               ) : (
-                <Outlet />
+                <RouteErrorBoundary>
+                  <Outlet />
+                </RouteErrorBoundary>
               )}
             </main>
             <PropertiesPanel />

ui/src/components/RouteErrorBoundary.tsx

@@ -0,0 +1,73 @@
+import { Component, type ErrorInfo, type ReactNode } from "react";
+import { useLocation, useNavigate } from "@/lib/router";
+import { Button } from "@/components/ui/button";
+
+type RouteErrorBoundaryInnerProps = {
+  resetKey: string;
+  onReset: () => void;
+  children: ReactNode;
+};
+
+type RouteErrorBoundaryState = {
+  error: Error | null;
+};
+
+class RouteErrorBoundaryInner extends Component<RouteErrorBoundaryInnerProps, RouteErrorBoundaryState> {
+  override state: RouteErrorBoundaryState = { error: null };
+
+  static getDerivedStateFromError(error: unknown): RouteErrorBoundaryState {
+    return { error: error instanceof Error ? error : new Error(String(error)) };
+  }
+
+  override componentDidCatch(error: unknown, info: ErrorInfo): void {
+    console.error("Page render failed", { error, componentStack: info.componentStack });
+  }
+
+  override componentDidUpdate(prevProps: RouteErrorBoundaryInnerProps): void {
+    // A render throw with no boundary unmounts the whole app, so navigating
+    // away (back button included) can't recover without a hard refresh. We sit
+    // above the routed <Outlet />, so reset whenever the route changes.
+    if (this.state.error && prevProps.resetKey !== this.props.resetKey) {
+      this.setState({ error: null });
+    }
+  }
+
+  override render() {
+    const { error } = this.state;
+    if (!error) return this.props.children;
+
+    return (
+      <div className="mx-auto max-w-2xl space-y-4 px-4 py-10">
+        <div>
+          <h1 className="text-lg font-semibold">This page hit an error</h1>
+          <p className="mt-1 text-sm text-muted-foreground">
+            Something went wrong while rendering this page. You can go back and try again, or reload.
+          </p>
+        </div>
+        <pre className="overflow-auto rounded-md border border-destructive/30 bg-destructive/10 px-3 py-2 text-xs text-destructive whitespace-pre-wrap">
+          {error.message}
+        </pre>
+        <div className="flex items-center gap-2">
+          <Button variant="outline" size="sm" onClick={this.props.onReset}>
+            Go back
+          </Button>
+          <Button size="sm" onClick={() => window.location.reload()}>
+            Reload page
+          </Button>
+        </div>
+      </div>
+    );
+  }
+}
+
+export function RouteErrorBoundary({ children }: { children: ReactNode }) {
+  const location = useLocation();
+  const navigate = useNavigate();
+  const resetKey = `${location.pathname}${location.search}`;
+
+  return (
+    <RouteErrorBoundaryInner resetKey={resetKey} onReset={() => navigate(-1)}>
+      {children}
+    </RouteErrorBoundaryInner>
+  );
+}