fix: do not make letterhead default

Author: khushi8112

erpnext/fixtures/letter_head.json

@@ -16,7 +16,7 @@
   "image": null,
   "image_height": 0.0,
   "image_width": 0.0,
-  "is_default": 1,
+  "is_default": 0,
   "letter_head_name": "Sales Invoice Letter Head",
   "modified": "2025-09-17 12:31:46.314509",
   "name": "Sales Invoice Letter Head",