Enhance ZenWelcome styling and functionality

- Add nowrap to welcome text for better layout
- Change button attribute to class for primary styling
- Update window centering logic to improve user experience

src/browser/base/content/zen-styles/zen-welcome.css

@@ -36,6 +36,7 @@
     line-height: 1.1;
     max-width: 50%;
     font-weight: 500;
+    white-space: nowrap;
 
     & > span {
       display: block;