Add Custom Buttons

[diemonic1]

Custom buttons

This plugin allows you to add additional custom buttons to the Steam client, which allow you to open any websites or apps on your computer - for example https://howlongtobeat.com/?q=%GAME_NAME% or S:\Programs\Epic Games\Launcher\Engine\Binaries\Win64\EpicGamesLauncher.exe

Old pull request #123 was automatically closed due to an error, so I'm opening PR again.
Plugin repository: https://github.com/diemonic1/Custom-buttons
This plugin allows you to add customizable buttons to the Steam interface.

Fixed

• Migrated to the starlight template (millennium.toml), TS errors 253 → 0, dependencies reduced to starlight + React types.
• Replaced deprecated AddWindowCreateHook / findElement and DOM cloning with Hooking API patches.
• Removed noise logs (backend and frontend).
• Language now follows the Steam client language; plugin language option removed.
• Removed own sleep.
• Top buttons: fixed CSS with 12px font; "Top buttons style" section removed in favour of QuickCSS (selectors documented in README).
• Settings rebuilt with Millennium components, IconsModule icons, description props; buttons are edited in a modal.

Not changed (and why)

• title on top buttons and app-page buttons: icon-only buttons have no other label, so the tooltip is the only way to know what a button does.
• Settings stay in localStorage: avoids migrating existing user data; obsolete keys are ignored.
• Text arrow in Store menu items (if confirmed): Steam's menu item name only accepts a string.
• Anything that turns out impossible during implementation is added here with the reason.

Task Checklist

Developer

• My plugin complies with the plugin submission guidelines
• 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.
• Generative AI was used for most of the plugin's code.

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

• I use a standard Millennium Lua backend in my plugin.
• I use custom binaries that or rely on other FOSS projects that aren't written directly using Millennium's Lua backend.

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.

In the old pull request there were answers that the plugin was being tested: #123

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Testing Instructions

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