fix(i18n): pass locale to background threads via thread-local storage

Background threads (graph building, simulation prep, report generation,
profile generation) now inherit the requesting user's locale preference.
Previously these fell back to 'zh' because Flask request context was
unavailable in spawned threads.

backend/app/utils/__init__.py

@@ -4,7 +4,7 @@
 
 from .file_parser import FileParser
 from .llm_client import LLMClient
-from .locale import t, get_locale, get_language_instruction
+from .locale import t, get_locale, set_locale, get_language_instruction
 
-__all__ = ['FileParser', 'LLMClient', 't', 'get_locale', 'get_language_instruction']
+__all__ = ['FileParser', 'LLMClient', 't', 'get_locale', 'set_locale', 'get_language_instruction']
 

backend/app/utils/locale.py

@@ -1,7 +1,10 @@
 import json
 import os
+import threading
 from flask import request, has_request_context
 
+_thread_local = threading.local()
+
 _locales_dir = os.path.join(os.path.dirname(__file__), '..', '..', '..', 'locales')
 
 # Load language registry
@@ -17,10 +20,15 @@ for filename in os.listdir(_locales_dir):
             _translations[locale_name] = json.load(f)
 
 
+def set_locale(locale: str):
+    """Set locale for current thread. Call at the start of background threads."""
+    _thread_local.locale = locale
+
+
 def get_locale() -> str:
     if has_request_context():
         return request.headers.get('Accept-Language', 'zh')
-    return 'zh'
+    return getattr(_thread_local, 'locale', 'zh')
 
 
 def t(key: str, **kwargs) -> str: