Fix LoRA training Error 500 - remove Radio serialization issue

The @gradio/client was wrapping Radio component values as
{"value": "replace", "__type__": "update"} instead of plain strings,
causing the update_settings Gradio call to fail and crash the server.

- /update-settings: removed Gradio call, settings are applied at save time
- /save-dataset: migrated to REST API (/v1/dataset/save) which accepts
  tag_position and other settings directly as strings
- Frontend: pass dataset settings (tag, position, etc.) when saving

services/api.ts

@@ -768,6 +768,10 @@ export const trainingApi = {
   saveDataset: (params: {
     savePath?: string;
     datasetName?: string;
+    customTag?: string;
+    tagPosition?: string;
+    allInstrumental?: boolean;
+    genreRatio?: number;
   }, token: string): Promise<{ status: string; path: string }> =>
     api('/api/training/save-dataset', { method: 'POST', body: params, token }),