fix(web): preserve session view on polling errors and add missing link mock

Polling errors no longer throw to the error boundary when a session is
already loaded, preventing transient network blips from unmounting the
view. Added missing next/link mock in error.test.tsx.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/web/src/app/error.test.tsx

@@ -7,6 +7,15 @@ vi.mock("next/navigation", () => ({
   useRouter: () => ({ refresh }),
 }));
 
+vi.mock("next/link", () => ({
+  default: ({
+    children,
+    ...props
+  }: React.PropsWithChildren<React.AnchorHTMLAttributes<HTMLAnchorElement>>) => (
+    <a {...props}>{children}</a>
+  ),
+}));
+
 import ErrorPage from "./error";
 
 describe("Route error boundary", () => {

packages/web/src/app/sessions/[id]/page.tsx

@@ -75,6 +75,7 @@ export default function SessionPage() {
   const sessionIsOrchestratorRef = useRef(false);
   const resolvedProjectSessionsKeyRef = useRef<string | null>(null);
   const prefixByProjectRef = useRef<Map<string, string>>(new Map());
+  const hasLoadedSessionRef = useRef(false);
 
   // Keep prefixByProjectRef in sync so fetchProjectSessions (stable [] dep) reads latest map
   useEffect(() => {
@@ -129,9 +130,12 @@ export default function SessionPage() {
       setSession(data);
       setRouteError(null);
       setSessionMissing(false);
+      hasLoadedSessionRef.current = true;
     } catch (err) {
       console.error("Failed to fetch session:", err);
-      setRouteError(err instanceof Error ? err : new Error("Failed to load session"));
+      if (!hasLoadedSessionRef.current) {
+        setRouteError(err instanceof Error ? err : new Error("Failed to load session"));
+      }
     } finally {
       setLoading(false);
     }