Pass the expected account email to the WP.com login webpage

[crazytonyli]

Note

This PR is built on top of #23749 and requires the changes in D165346-code.

When re-authenticating or signing in to a Jetpack site, we expected a specific WP.com account. We already have checks in place to throw an error if users sign in with an unexpected WP.com account. However, from UX perspective, it'd be better to pre-fill the username/email text field on the login page with the expected WP.com account email. That's what this PR does.

Test Instructions

Use Jetpack login as an example.

1. On desktop browser, fully connect a self-hosted site to Jetpack.
2. On the mobile app, add the self-hosted site.
3. Go to "Stats" in the app, and tap "Log in"

The username/email text field in the login web page should be prepopulated with the email addresses of the account used in step 1.

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 25.6. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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[wpmobilebot]

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

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23766-aef1fa8
	Version	25.4.2
	Bundle ID	org.wordpress.alpha
	Commit	aef1fa8
	App Center Build	WPiOS - One-Offs #11123

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

[wpmobilebot]

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

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23766-aef1fa8
	Version	25.4.2
	Bundle ID	com.jetpack.alpha
	Commit	aef1fa8
	App Center Build	jetpack-installable-builds #10163

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