Ibis: Update header defaults and screenshot to match

- Update header to be a bit more compact with a custom font size.
- Add a screenshot to match the front end.

This should get Ibis to a state where it's good enough to start testing on WP.com with our Starter Designs and Patterns.

ibis/block-template-parts/header.html

@@ -1,7 +1,11 @@
 <!-- wp:group {"align":"full","backgroundColor":"background"} -->
-<div class="wp-block-group alignfull has-background-background-color has-background"><div class="wp-block-group__inner-container"><!-- wp:columns {"align":"wide"} -->
+<div class="wp-block-group alignfull has-background-background-color has-background"><div class="wp-block-group__inner-container"><!-- wp:spacer {"height":20} -->
+<div style="height:20px" aria-hidden="true" class="wp-block-spacer"></div>
+<!-- /wp:spacer -->
+
+<!-- wp:columns {"align":"wide"} -->
 <div class="wp-block-columns alignwide"><!-- wp:column {"width":33.33} -->
-<div class="wp-block-column" style="flex-basis:33.33%"><!-- wp:site-title /--></div>
+<div class="wp-block-column" style="flex-basis:33.33%"><!-- wp:site-title {"style":{"typography":{"fontSize":32}}} /--></div>
 <!-- /wp:column -->
 
 <!-- wp:column {"width":66.66} -->