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/Modules/Settings/DankBarTab.qml

@@ -714,6 +714,15 @@ Item {
                     })
             }
 
+            SettingsToggleCard {
+                iconName: "filter_b_and_w"
+                title: I18n.tr("Monochrome System Tray Icons")
+                description: I18n.tr("Desaturate all system tray icons for a uniform monochrome look")
+                visible: selectedBarConfig?.enabled
+                checked: SettingsData.systemTrayMonochromeIcons
+                onToggled: checked => SettingsData.set("systemTrayMonochromeIcons", checked)
+            }
+
             SettingsToggleCard {
                 iconName: "mouse"
                 title: I18n.tr("Scroll Wheel")