feat(windows): add global overlay opacity control

[silvermoon89]

Summary

• add a 0-100% overlay opacity slider to the Windows tray menu
• apply one theme-backed opacity value to the titlebar, sidebar, and immersive content surfaces
• preserve the value when importing wallpapers or switching saved themes
• include overlayOpacity in the validated renderer payload so watcher updates apply without restarting Codex
• allow the tray menu builder to accept its initially empty item collection

This PR is independently mergeable. If #93 is also merged, the settings route automatically uses the same global CSS variable.

Tests

• node windows/tests/renderer-inject.test.mjs
• node windows/tests/injector-bootstrap.test.mjs
• node windows/tests/injector-one-shot.test.mjs
• node windows/scripts/injector.mjs --check-payload
• powershell.exe -NoProfile -ExecutionPolicy Bypass -File windows/tests/run-tests.ps1
• git diff --check

[Fei-Away]

Thanks for the contribution! We just merged #216, unifying the macOS/Windows skin runtime into a single source (runtime/ + tools/selectors.json, attribute-driven rendering) — it rewrote the files this PR touches, so it's no longer compatible as submitted. Closing for now; a fresh PR against the new runtime would be very welcome if the idea still applies. Thanks again for the time you put into this!

感谢贡献！我们刚合并了 #216，把双端换肤 runtime 统一成了单一源码（runtime/ + tools/selectors.json，属性驱动渲染），这个 PR 改动的文件已经被那次重构重写，现在不再适用。先关闭，如果思路还有价值，非常欢迎基于新架构重新提一个 PR。谢谢你花的时间！