集成 MyTools：启动拉取配置、动态加班起算时间、保存加班记录

- 启动时从 MyTools 拉取加班起算时间配置（1s超时fallback默认17:20）
- calculateTimeDifference 改用动态 overtimeStart 替代硬编码 17:20
- 计算完成后显示"保存到 MyTools"按钮，POST 记录到 API
- 支持 upsert（同日期覆盖），Toast 提示成功/失败
- MyTools 不可用时所有功能独立正常运行

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

holidays_2026.json

@@ -5419,8 +5419,8 @@
         "holiday": 44,
         "holiday_or": 45,
         "holiday_overtime": 10,
-        "holiday_today": 2,
-        "holiday_legal": 2,
+        "holiday_today": 1,
+        "holiday_legal": 1,
         "holiday_recess": 1
       },
       {
@@ -5439,8 +5439,8 @@
         "holiday": 44,
         "holiday_or": 44,
         "holiday_overtime": 10,
-        "holiday_today": 1,
-        "holiday_legal": 1,
+        "holiday_today": 2,
+        "holiday_legal": 2,
         "holiday_recess": 1
       },
       {