[Jetpack setup] UI updates for account creation support

[hichamboushaba]

Closes: #14421

Description

This PR just adds some UI changes for better support of the account creation flow:

• Adds a note to the email screen about account creation.
• Updates the error message if the user uses an unknown username to indicate that they should enter an email instead.
• Adds a note to the magic link screen about confirming the correct email.

Steps to reproduce

1. Sign in using Application Passwords.
2. Open the Jetpack setup flow from the dashboard.

Testing information

• Confirm the presence of the new note on the email screen.
• Enter an unknown username and confirm the new error message is shown.
• Start magic link flow (either signup or login) and confirm that the screen has the new note.

Screenshots

Email screen	Username error	Magic Link screen

---

• [ ] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• [ ] The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• [ ] Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• [ ] Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[dangermattic]

	1 Warning
:warning:	This PR is assigned to the milestone 21.2. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by :no_entry_sign: Danger

[wpmobilebot]

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

	App Name	WooCommerce iOS
	Build Number	pr14454-059d0f0
	Version	21.1
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	059d0f03376e8d53ec36252d5ec35b3482f0e9b4
	App Center Build	WooCommerce - Prototype Builds #11709

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