Add self-contained Windows wallpaper manager

[CCDawn]

What changed

• Adds a self-contained .NET 8 WinForms manager for Codex Dream Skin.
• Embeds the tested Dream Skin PowerShell/JavaScript engine and a Node.js runtime into one portable Windows executable.
• Adds wallpaper library browsing/search, image and video preview, wallpaper switching, reveal control, pause/resume, restore, tray controls, and optional per-user startup.
• Adds a reproducible release build script, manager command bridge, documentation, changelog entry, and regression assertions.

User impact

Windows users can now manage Codex themes and internal image/video wallpapers from a single EXE without installing Node.js, the .NET SDK, or running PowerShell manually.

Validation

• powershell.exe -NoProfile -ExecutionPolicy Bypass -File windows/tests/run-tests.ps1
• Self-contained win-x64 single-file publish
• Published EXE --self-test
• Manual Windows UI validation of library loading, MP4 preview, layout, and runtime startup