chore: preset factory update source in default config (internal use)

Ship update_source=http://cm.xiapi.com with the internal read account so a
fresh install is preconfigured (seeded into ~/.cmbot on first run). Plaintext
credentials are acceptable for internal LAN use per project decision.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/default_config/app_config.json

@@ -6,7 +6,7 @@
   "last_print_dir": "",
   "last_template": "",
   "last_batch_mode": "full_combo",
-  "update_source": "",
-  "update_user": "",
-  "update_pass": ""
+  "update_source": "http://cm.xiapi.com",
+  "update_user": "admin",
+  "update_pass": "chengma"
 }