fix(shell-panel): handle overflow scrolling (#12069)

**Related Issue:** #12057

## Summary

- add full width and height to container
- add screenshot test
- repro test case: https://codepen.io/driskull/pen/xbbPBwN?editors=1000
- Notice how the center bottom shell panel isn't positioned or
overflowing correctly

Author: driskull

packages/calcite-components/src/components/shell-panel/shell-panel.scss

@@ -283,7 +283,9 @@
   items-stretch
   self-stretch
   relative
-  box-border;
+  box-border
+  w-full
+  h-full;
 }
 
 :host([layout="horizontal"]) .content-container {