feat: add support for geometric centering (#856)

Introduces a configurable centering mode.
- Adds 'geometric' option.
- Retains 'index' as the default value to preserve existing behavior.

quickshell/Common/SettingsData.qml

@@ -175,6 +175,7 @@ Singleton {
     property bool keyboardLayoutNameCompactMode: false
     property bool runningAppsCurrentWorkspace: false
     property bool runningAppsGroupByApp: false
+    property string centeringMode: "index"
     property string clockDateFormat: ""
     property string lockDateFormat: ""
     property int mediaSize: 1
@@ -227,6 +228,7 @@ Singleton {
     onNotepadFontFamilyChanged: saveSettings()
     onNotepadFontSizeChanged: saveSettings()
     onNotepadShowLineNumbersChanged: saveSettings()
+    // onCenteringModeChanged: saveSettings()
     onNotepadTransparencyOverrideChanged: {
         if (notepadTransparencyOverride > 0) {
             notepadLastCustomTransparency = notepadTransparencyOverride;

quickshell/Common/settings/SettingsSpec.js

@@ -90,6 +90,7 @@ var SPEC = {
     keyboardLayoutNameCompactMode: { def: false },
     runningAppsCurrentWorkspace: { def: false },
     runningAppsGroupByApp: { def: false },
+    centeringMode: { def: "index" },
     clockDateFormat: { def: "" },
     lockDateFormat: { def: "" },
     mediaSize: { def: 1 },