Fix calculate_attendance_status missing return result, move chart imports to top

backend/main.py

@@ -309,6 +309,7 @@ def calculate_attendance_status(check_in, check_out, shift_start, shift_end):
     result["status_text"] = " / ".join(status_parts) if status_parts else "正常"
     
 
+    return result
 @app.get("/api/attendance/lateness")
 async def get_lateness_stats(current_user: User = Depends(get_current_user)):
     """Calculate full attendance statistics: late, early, OT"""