feat(desktop): redirect writable data to ~/.cow for packaged app

Introduce get_data_root() driven by the COW_DATA_DIR env var so the
packaged desktop build stores config.json, run.log, user data and
WeChat credentials under ~/.cow — surviving app updates and keeping the
app bundle read-only. Source deployments leave COW_DATA_DIR unset and
fall back to the repo root, so existing behavior is unchanged.

common/cloud_client.py

@@ -21,7 +21,7 @@ from bridge.context import Context, ContextType
 from bridge.reply import Reply, ReplyType
 from common.log import logger
 from linkai import LinkAIClient, PushMsg
-from config import conf, pconf, plugin_config, available_setting, write_plugin_config, get_root
+from config import conf, pconf, plugin_config, available_setting, write_plugin_config, get_root, get_weixin_credentials_path
 from plugins import PluginManager
 import threading
 import time
@@ -336,9 +336,7 @@ class CloudClient(LinkAIClient):
     @staticmethod
     def _remove_weixin_credentials():
         """Remove the weixin token credentials file so next connect triggers QR login."""
-        cred_path = os.path.expanduser(
-            conf().get("weixin_credentials_path", "~/.weixin_cow_credentials.json")
-        )
+        cred_path = get_weixin_credentials_path()
         try:
             if os.path.exists(cred_path):
                 os.remove(cred_path)