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/api/report.py

@@ -15,7 +15,7 @@ from ..services.simulation_manager import SimulationManager
 from ..models.project import ProjectManager
 from ..models.task import TaskManager, TaskStatus
 from ..utils.logger import get_logger
-from ..utils.locale import t
+from ..utils.locale import t, get_locale, set_locale
 
 logger = get_logger('mirofish.api.report')
 
@@ -121,8 +121,12 @@ def generate_report():
             }
         )
         
+        # Capture locale before spawning background thread
+        current_locale = get_locale()
+
         # 定义后台任务
         def run_generate():
+            set_locale(current_locale)
             try:
                 task_manager.update_task(
                     task_id,