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.

voice/dashscope/dashscope_voice.py

@@ -12,6 +12,7 @@ from dashscope import MultiModalConversation
 
 from bridge.reply import Reply, ReplyType
 from common.log import logger
+from common.tmp_dir import TmpDir
 from config import conf
 from voice import audio_convert
 from voice.voice import Voice
@@ -121,8 +122,7 @@ class DashScopeVoice(Voice):
     @staticmethod
     def _download_audio(url: str) -> Optional[str]:
         try:
-            tmp_dir = os.path.join(os.getcwd(), "tmp")
-            os.makedirs(tmp_dir, exist_ok=True)
+            tmp_dir = TmpDir().path()
             ts = datetime.datetime.now().strftime("%Y%m%d%H%M%S")
             ext = os.path.splitext(url.split("?", 1)[0])[1].lower() or ".wav"
             if ext not in (".mp3", ".wav", ".m4a", ".aac", ".opus"):