fix(ux): modal error dialog for server-refused actions + ESC closes only the topmost

- Duplicate-tag create/rename hit the uq_tags_user_name constraint and 500'd; now caught and
  returned as a clear 409 ('You already have a tag named …').
- New global error dialog (errorDialog store + ErrorDialog modal) for definitive server refusals;
  the api layer wiring + mount land with the rest of the round.
- Modal now keeps a stack so ESC dismisses only the top modal — an error over the tag editor
  closes the error and leaves the editor open.

frontend/src/components/ErrorDialog.tsx

@@ -0,0 +1,24 @@
+import { useSyncExternalStore } from "react";
+import { useTranslation } from "react-i18next";
+import { dismissError, getError, subscribeError } from "../lib/errorDialog";
+import Modal from "./Modal";
+
+// Renders the current app error (if any) as a modal the user must acknowledge.
+export default function ErrorDialog() {
+  const { t } = useTranslation();
+  const err = useSyncExternalStore(subscribeError, getError, getError);
+  if (!err) return null;
+  return (
+    <Modal title={err.title} onClose={dismissError} maxWidth="max-w-md">
+      <p className="text-sm leading-relaxed text-fg/90">{err.message}</p>
+      <div className="flex justify-end mt-5">
+        <button
+          onClick={dismissError}
+          className="px-4 py-2 rounded-lg bg-accent text-accent-fg text-sm font-medium hover:opacity-90 transition"
+        >
+          {t("errors.ok")}
+        </button>
+      </div>
+    </Modal>
+  );
+}