fix(windows): load XmlDocument WinRT type before use

[mark-nhk]

Thanks for the extension — the sound alerts already solved most of my 'did Claude finish?' problem. This PR fixes the banner half on Windows.

Problem

On Windows, sound notifications work but the system banner never appears. The extension's own diagnostics pinpoint the failure:

FAIL 1/2 System banner: Command failed: powershell -NoProfile -Command [...] New-Object : Cannot find type [Windows.Data.Xml.Dom.XmlDocument]: verify that the assembly containing this type is loaded.

OK 2/2 Sound: Command ran without error

Cause

In Windows PowerShell 5.1, each WinRT type must be loaded with the [Type, Assembly, ContentType=WindowsRuntime] syntax before first use. The toast command in lib/system-notification.js only loads ToastNotificationManager, so New-Object Windows.Data.Xml.Dom.XmlDocument fails, which cascades into the two follow-up errors (LoadXml on null, missing ToastNotification constructor).

This affects all Windows users of the extension, not a specific machine configuration.

Fix

Add the missing type-load statement for Windows.Data.Xml.Dom.XmlDocument before it is instantiated. One line, no behavior change on macOS/Linux.

Verified

Windows 11, Windows PowerShell 5.1:

• Reproduced the failure by running the original command manually — same three errors as the diagnostics.
• Ran the command with the added type-load line — the banner appears.
• (Side note: the snippet only works in Windows PowerShell 5.1; PowerShell 7 cannot load WinRT types this way at all. The extension correctly invokes powershell, so this is fine — just worth knowing for future changes.)