Improve WebView2 bootstrap recovery

[Watanabehato]

Summary

• add an early Windows bootstrap log before Tauri logging starts
• mark WebView startup as pending and rotate cache/webview_data on the next launch if Tauri setup was never reached
• avoid using an incomplete fixed WebView2 runtime and fall back to system/runtime install instead

Testing

• git diff --check
• Not run: cargo fmt/check, cargo is not available in this environment

Summary by Sourcery

提升 Windows WebView2 启动的健壮性和诊断能力，以从启动失败中恢复，并避免使用不完整的固定运行时。

New Features:

• 在 Tauri 日志初始化之前，将早期的 Windows 启动日志记录到 debug/bootstrap.log 文件中。
• 使用标记文件跟踪 WebView 启动状态，如果从未到达 Tauri setup 阶段，则在下次启动时触发缓存的 webview_data 轮换。

Bug Fixes:

• 当缓存的固定 WebView2 运行时缺少必需文件时，跳过使用该缓存，改为回退到系统或运行时安装。
• 通过备份并重新创建 webview_data 目录，从之前失败的 WebView 启动中恢复。

Enhancements:

• 统一 Windows 可执行文件目录解析逻辑，并在各个启动辅助模块中复用。
• 确保记录 WebView2 用户数据和运行时选择，以帮助排查启动相关问题。

Original summary in English

Summary by Sourcery

Improve Windows WebView2 startup robustness and diagnostics to recover from failed boots and avoid using incomplete fixed runtimes.

New Features:

• Add early Windows bootstrap logging to a debug/bootstrap.log file before Tauri logging initializes.
• Track WebView startup state with a marker file to trigger rotation of cached webview_data on the next launch if Tauri setup is never reached.

Bug Fixes:

• Skip using a cached fixed WebView2 runtime when required files are missing and fall back to system or runtime installation instead.
• Recover from previous failed WebView boots by backing up and recreating the webview_data directory.

Enhancements:

• Centralize Windows executable directory resolution and reuse it across bootstrap helpers.
• Ensure WebView2 user data and runtime selection are logged to aid troubleshooting of bootstrap issues.