fix: handle empty xtream categories gracefully

Skip database insert when categories array is empty to prevent
'values() must be called with at least one value' error.

Closes #684

apps/electron-backend/src/app/events/database/category.events.ts

@@ -75,6 +75,11 @@ ipcMain.handle(
         type: 'live' | 'movies' | 'series'
     ) => {
         try {
+            // Skip if no categories to save
+            if (!categories || categories.length === 0) {
+                return { success: true };
+            }
+
             const db = await getDatabase();
             const values = categories.map((cat) => ({
                 playlistId,