feat(system-tray): add optional monochrome icons setting (#2241)

Adds a 'Monochrome Icons' toggle to the system tray widget context menu.
When enabled, all system tray icons are desaturated using MultiEffect,
giving a cleaner monochrome bar aesthetic that matches minimal themes.

The setting is per-user (settings.json), defaults to false to preserve
existing behavior.

quickshell/Common/SettingsData.qml

@@ -211,6 +211,7 @@ Singleton {
     property int selectedGpuIndex: 0
     property var enabledGpuPciIds: []
     property bool showSystemTray: true
+    property bool systemTrayMonochromeIcons: false
     property bool showClock: true
     property bool showNotificationButton: true
     property bool showBattery: true

quickshell/Common/settings/SettingsSpec.js

@@ -79,6 +79,7 @@ var SPEC = {
     selectedGpuIndex: { def: 0 },
     enabledGpuPciIds: { def: [] },
     showSystemTray: { def: true },
+    systemTrayMonochromeIcons: { def: false },
     showClock: { def: true },
     showNotificationButton: { def: true },
     showBattery: { def: true },