- Add emails/admin_compose.html: branded wrapper for ad-hoc compose body
- Update email_compose.html: two-column layout with HTMX live preview pane
(hx-post, hx-trigger=input delay:500ms, hx-target=#preview-pane)
- Add partials/email_preview_frame.html: sandboxed iframe partial
- Add POST /admin/emails/compose/preview route (no CSRF — read-only render)
- Update email_compose POST handler to use render_email_template() instead
of importing _email_wrap from worker
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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