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/simulation.py

@@ -14,7 +14,7 @@ from ..services.oasis_profile_generator import OasisProfileGenerator
 from ..services.simulation_manager import SimulationManager, SimulationStatus
 from ..services.simulation_runner import SimulationRunner, RunnerStatus
 from ..utils.logger import get_logger
-from ..utils.locale import t
+from ..utils.locale import t, get_locale, set_locale
 from ..models.project import ProjectManager
 
 logger = get_logger('mirofish.api.simulation')
@@ -501,8 +501,12 @@ def prepare_simulation():
         state.status = SimulationStatus.PREPARING
         manager._save_simulation_state(state)
         
+        # Capture locale before spawning background thread
+        current_locale = get_locale()
+
         # 定义后台任务
         def run_prepare():
+            set_locale(current_locale)
             try:
                 task_manager.update_task(
                     task_id,