CCM-10630: Give error summary focus on appearance & adjust letter errors

[ClareJonesBJSS]

Description

• Added focus behaviour to ZodErrorSummary and renamed to encourage universal use.
• Removed errorHeading variable as according to guidance it should always be 'There is a problem'
• Switched deprecated <zodError>.flatten() calls to z.flattenError(<zodError>)
• Replaced ErrorMessage use for messages not tied to a specific field, to eliminate unwanted hidden 'Error: ' and correctly nest in <li>s

Context

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.