Fix backtest validation and improve error messaging

[htazq]

📝 Description | 描述

English: Fix backtest crash when decision_timeframe is not included in timeframes; surface sanitized error messages and keep UI in sync.
中文： 修复回测在决策周期不包含于周期列表时的崩溃问题；改进错误提示并确保前端配置一致。

---

🎯 Type of Change | 变更类型

• 🐛 Bug fix | 修复 Bug

---

🔗 Related Issues | 相关 Issue

• Related to # (none)

---

📋 Changes Made | 具体变更

English:

• Validate decision_timeframe in backtest config and add safe guards in DataFeed to avoid nil panic; return sanitized error to client.
• Keep decision timeframe in sync with selected timeframes in UI; improve user-facing error hints.

中文：

• 后端校验 decision_timeframe 必须包含在 timeframes 中，并在 DataFeed 增加空指针保护；将安全的具体错误返回前端。
• 前端选择周期时自动同步决策周期，避免非法配置；完善用户错误提示。

---

🧪 Testing | 测试

• Tested locally | 本地测试通过
• Tests pass | 测试通过
• Verified no existing functionality broke | 确认没有破坏现有功能

---

✅ Checklist | 检查清单

Code Quality | 代码质量

• Code follows project style | 代码遵循项目风格
• Self-review completed | 已完成代码自查
• Comments added for complex logic | 已添加必要注释

Documentation | 文档

• Updated relevant documentation | 已更新相关文档

Git

• Commits follow conventional format | 提交遵循 Conventional Commits 格式
• Rebased on latest dev branch | 已 rebase 到最新 dev 分支
• No merge conflicts | 无合并冲突

---

📚 Additional Notes | 补充说明

English: Tests not run locally.
中文： 尚未在本地运行测试。

---

By submitting this PR, I confirm | 提交此 PR，我确认：

• I have read the Contributing Guidelines | 已阅读贡献指南
• I agree to the Code of Conduct | 同意行为准则
• My contribution is licensed under AGPL-3.0 | 贡献遵循 AGPL-3.0 许可证

[cla-assistant]

All committers have signed the CLA.

[htazq]

Tests not run locally. Please let me know if you want me to run any specific tests.