feat(system-tray): add optional monochrome icons setting

[ronilaukkarinen]

Summary

Adds an opt-in "Monochrome System Tray Icons" toggle under Settings → Dank Bar → Settings (right after "Maximize Detection"). When enabled, every system tray icon is desaturated via MultiEffect { saturation: -1 }, producing a uniform monochrome bar that matches minimal / one-color themes.

Default is false, so existing users see no change.

Changes

• quickshell/Common/settings/SettingsSpec.js - add systemTrayMonochromeIcons: { def: false }
• quickshell/Common/SettingsData.qml - add property bool systemTrayMonochromeIcons: false
• quickshell/Modules/DankBar/Widgets/SystemTrayBar.qml - wire layer.enabled + MultiEffect with saturation: -1 on the four IconImage blocks (horizontal bar, vertical bar, overflow menu item, inline expansion icon)
• quickshell/Modules/Settings/DankBarTab.qml - new SettingsToggleCard after "Maximize Detection"

Test plan

• Toggle in Settings → Dank Bar appears after "Maximize Detection"
• Enabling the toggle desaturates all tray icons (Signal, 1Password, Spotify, Element, etc.) to grayscale
• Disabling restores color
• Overflow menu icons also respect the setting

Disabled (default):

Enabled (toggled):

Icon: https://fonts.google.com/icons?selected=Material+Symbols+Rounded:filter_b_and_w

[arora-archit]

i think apart from monochrome, having an option to choose filter from primary, secondary, surface colours etc. will be nice to have