Update header styling of consolidated list

Summary

This issue is to account for the updates to the header styling of the consolidated list, as shown in the initial designs in #430137 (closed). This was discussed in !213513 (comment 2902236043) and broken out as a follow-up.

Proposal

• Include a background (background-color-subtle) for the entire header area instead of just the filter/display options area.
• Ensure page-level Alerts also appear in this area when they are shown (see designs below).
• Header background should go to edge of panel/page
• Header should have a border-bottom (border-color-subtle)
• Sticky header should reflect these updates
• In the case the List isn't fluid width (i.e. if !209478 (merged) hasn't merged), the content in the header should abide to the limited container, while the header maintains full width (see designs below).

Resources

• 🔍 View in Figma