fix(build): fallback to logo.png when logo_mark.png is unavailable - #13

[中文]
- 变更内容：
  - MSIX 资产生成时增加图标回退逻辑，优先使用 `logo_mark.png`，缺失时自动回退 `logo.png`。
- 影响范围：
  - `scripts/build_msix.ps1` 图标准备阶段。
- 兼容性/行为变化：
  - 不再因 `logo_mark.png` 缺失导致构建失败；其余流程保持不变。
- 依赖/环境：
  - 无新增依赖。

[English]
- Changes:
  - Added icon fallback in MSIX asset generation: prefer `logo_mark.png`, fallback to `logo.png`.
- Impact:
  - `scripts/build_msix.ps1` icon preparation stage.
- Compatibility / Behavior Changes:
  - Build no longer fails when `logo_mark.png` is missing; other behavior remains unchanged.
- Dependencies / Environment:
  - No new dependency.