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/graph.py

@@ -15,7 +15,7 @@ from ..services.graph_builder import GraphBuilderService
 from ..services.text_processor import TextProcessor
 from ..utils.file_parser import FileParser
 from ..utils.logger import get_logger
-from ..utils.locale import t
+from ..utils.locale import t, get_locale, set_locale
 from ..models.task import TaskManager, TaskStatus
 from ..models.project import ProjectManager, ProjectStatus
 
@@ -371,8 +371,12 @@ def build_graph():
         project.graph_build_task_id = task_id
         ProjectManager.save_project(project)
         
+        # Capture locale before spawning background thread
+        current_locale = get_locale()
+
         # 启动后台任务
         def build_task():
+            set_locale(current_locale)
             build_logger = get_logger('mirofish.build')
             try:
                 build_logger.info(f"[{task_id}] 开始构建图谱...")