Add SteamWindowsToastNotification plugin

[GID0317]

SteamWindowsToastNotifiy

SteamWindowsToastNotification replaces Steam desktop popup notifications with native Windows notifications.

It supports common Steam notifications such as friend messages, group chat messages, friend online alerts, voice chat requests, game invites, download complete alerts, and tutorial prompts. Where Steam exposes matching actions, the plugin forwards them to Windows notification actions, such as opening chat, joining a game, or dismissing a call.

Task Checklist

Developer

• I am the original author or an authorized maintainer of this plugin.
• I have complied with all license requirements for the libraries used, including providing appropriate notices where necessary.
• My plugin is fully open source and does not depend on any external paid services, except for widely trusted and well-known platforms. Additionally, neither I nor anyone associated with me profits from any such services.

Plugin Functionality

• I have tested the plugin on both the Stable and Beta Steam update channels.
• My plugin is unique, or provides additional or alternative functionality to plugins already on the store.

Backend Configuration

• No: I use a standard Millennium python backend in my plugin.
• No: I use custom binaries that or rely on other FOSS projects that aren't written directly using Millennium's python backend.

Backend note: this plugin uses Millennium's Lua backend and a TypeScript/React frontend. It does not bundle custom binaries. Windows notification delivery uses built-in Windows PowerShell/WinRT APIs.

Community Contribution

• I have tested and left feedback on two other plugin pull requests.
• I have added links to those feedback comments in this PR.

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Testing Instructions

• Verified by a third party on Steam Client Stable.
• Verified by a third party on Steam Client Beta.