Center the panel menu under the button

The button was created with menuAlignment 0.0, which places the arrow at
the menu's left edge while it still points at the button's center, so the
menu opened from the middle of the widget. Use 0.5 to match the default
source alignment and center the menu under the button like other indicators.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/indicator.js

@@ -17,7 +17,7 @@ import { formatSpeed, formatSpeedShort } from './format.js';
 export const SingBoxIndicator = GObject.registerClass(
 class SingBoxIndicator extends PanelMenu.Button {
     _init(extension) {
-        super._init(0.0, 'sing-box Status', false);
+        super._init(0.5, 'sing-box Status', false);
 
         this._extension = extension;
         this._settings = extension.getSettings();