Reader: Add separator to site header

[kean]

There is clearly a need for a separator between the site header and the list of posts.

Before / After

iPad

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 26.8. This milestone is due in less than 4 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by 🚫 Danger

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	31579
	Version	PR #25403
	Bundle ID	com.jetpack.alpha
	Commit	f289eb4
	Installation URL	2gcrhm6b986v8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	31579
	Version	PR #25403
	Bundle ID	org.wordpress.alpha
	Commit	f289eb4
	Installation URL	1eub24b3m7ld0

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[crazytonyli]

Can any of these existing APIs be used here?