Clicking an embedded email attachment (bounce/DSN, forward-as-attachment, ...)
opened only a download. Add an 'eml' preview kind: FilePreviewModal parses the
blob with postal-mime (dynamic-imported, off the bundle) and renders it via a
new EmlPreview component - header (from/to/subject/date) + body + the message's
own attachments.
The body is sanitized with DOMPurify (sanitizeEmailHtmlForIframe) AND rendered
in a fully-locked sandbox iframe (sandbox="" - no scripts, no same-origin), so a
script-bearing .eml can never execute in-origin. Reuses the email_viewer locale
namespace (no new keys).
