v3: error cards across the app, friendly network errors, code quality pass

src/components/AppHeader.tsx

@@ -4,7 +4,9 @@ import { useAuth } from "../hooks/useAuth.ts";
 import { DumpCreateModal } from "./DumpCreateModal.tsx";
 import { NotificationBell } from "./NotificationBell.tsx";
 
-export function AppHeader({ centerSlot }: { centerSlot?: ReactNode }) {
+export function AppHeader(
+  { centerSlot, disableNew }: { centerSlot?: ReactNode; disableNew?: boolean },
+) {
   const { user } = useAuth();
   const navigate = useNavigate();
   const headerRef = useRef<HTMLElement>(null);
@@ -55,6 +57,8 @@ export function AppHeader({ centerSlot }: { centerSlot?: ReactNode }) {
                   type="button"
                   className="btn-primary"
                   onClick={() => setCreateModalOpen(true)}
+                  disabled={disableNew}
+                  title={disableNew ? "Server unreachable" : undefined}
                 >
                   + New
                 </button>