The inline HtmlAttachmentPreview toolbar carries an "Open in new tab"
button that routes to /{slug}/attachments/{id}/preview. The full-screen
AttachmentPreviewModal was missing the same affordance, so users who
maximized an HTML preview lost the ability to pop it into its own tab.
Mirror the gating exactly: show when kind === 'html' && slug &&
attachmentId. Other PreviewKinds keep the existing header (Download +
Close) — they don't have a corresponding full-page route.
Co-authored-by: multica-agent <github@multica.ai>