fix: correct default API_BASE_URL and improve sidebar error message

Agent-Logs-Url: https://github.com/velocitatem/cvfs/sessions/1a9aa3e0-c4e1-48fb-b646-49f31c316325

Co-authored-by: velocitatem <60182044+velocitatem@users.noreply.github.com>

apps/webapp/src/app/dashboard/page.tsx

@@ -495,7 +495,7 @@ export default function Dashboard() {
                 setDocs(d);
                 if (d.length) { setSelectedDocId(d[0].id); setSelectedVersionId(d[0].root_version_id ?? null); }
             })
-            .catch(e => setError(e.message))
+            .catch(() => setError('Failed to load documents. Make sure the backend is running.'))
             .finally(() => setLoading(false));
     }, []);