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/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 },