feat(Settings): add Settings window opacity option

[A1ca7raz]

• New "Settings Window Opacity" slider in Theme & Colors > Modal Background
• Adds settingsModalOpacity setting (default 100%, 0-100% range)
• Settings modal background honors the opacity value
• Make sidebar transparent so window opacity shows through
• Remove redundant title bar Rectangle that caused opacity desync

Description

Add a "Settings Window Opacity" slider (0–100%) in Theme & Colors that makes the Settings window see through to the desktop with a surface-color tint.

For niri users: disable shadow.draw-behind-window of settings modal to avoid unexpected dark background.

window-rule {
    match app-id="^com.danklinux.dms$" title="Settings|设置"
    open-floating true
    shadow {
        draw-behind-window false
    }
}

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Refactor / internal cleanup
• Documentation
• Other

Related issues

Screenshots / video

• 100% Opacity

• 60% Opacity

• 0% Opacity (Not recommended)

Checklist

• My code follows the conventions in CONTRIBUTING.md
• I have tested my changes locally
• New user-facing strings are wrapped in I18n.tr() with translator context, reusing existing terms where possible
• Go changes: ran make fmt, added/updated tests, make test passes, and go mod tidy is clean
• QML changes: ran make lint-qml with no new warnings
• I have opened a corresponding pull request in dlx-docs to document any new behaviors: https://github.com/AvengeMedia/DankLinux-Docs

[bbedward]

We pretty much got rid of this with FloatingWindow because the compositor can simply do it better. I feel like we could tie it to the dms window rule management, for compositors that are supported by that