feat(emails): subtask 5 — compose preview (admin_compose template + HTMX endpoint)

- 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>

web/src/padelnomics/admin/routes.py

@@ -33,6 +33,7 @@ from ..core import (
     utcnow,
     utcnow_iso,
 )
+from ..email_templates import EMAIL_TEMPLATE_REGISTRY, render_email_template
 
 logger = logging.getLogger(__name__)
 
@@ -1398,10 +1399,16 @@ async def email_compose():
                 email_addresses=EMAIL_ADDRESSES,
             )
 
-        html = f"<p>{body.replace(chr(10), '<br>')}</p>"
+        body_html = f"<p>{body.replace(chr(10), '<br>')}</p>"
         if wrap:
-            from ..worker import _email_wrap
-            html = _email_wrap(html)
+            html = render_email_template(
+                "emails/admin_compose.html",
+                lang="en",
+                body_html=body_html,
+                preheader="",
+            )
+        else:
+            html = body_html
 
         result = await send_email(
             to=to, subject=subject, html=html,
@@ -1424,6 +1431,36 @@ async def email_compose():
     )
 
 
+@bp.route("/emails/compose/preview", methods=["POST"])
+@role_required("admin")
+async def compose_preview():
+    """HTMX endpoint: render live preview for compose textarea (no CSRF — read-only)."""
+    form = await request.form
+    body = form.get("body", "").strip()
+    wrap = form.get("wrap", "") == "1"
+
+    body_html = f"<p>{body.replace(chr(10), '<br>')}</p>" if body else ""
+
+    if wrap and body_html:
+        try:
+            rendered_html = render_email_template(
+                "emails/admin_compose.html",
+                lang="en",
+                body_html=body_html,
+                preheader="",
+            )
+        except Exception:
+            logger.exception("compose_preview: template render failed")
+            rendered_html = body_html
+    else:
+        rendered_html = body_html
+
+    return await render_template(
+        "admin/partials/email_preview_frame.html",
+        rendered_html=rendered_html,
+    )
+
+
 # --- Audiences ---
 
 @bp.route("/emails/audiences")

web/src/padelnomics/admin/templates/admin/email_compose.html

@@ -2,14 +2,28 @@
 {% set admin_page = "compose" %}
 {% block title %}Compose Email - Admin - {{ config.APP_NAME }}{% endblock %}
 
+{% block admin_head %}
+<style>
+  .compose-layout { display: grid; grid-template-columns: 480px 1fr; gap: 1.5rem; align-items: start; }
+  @media (max-width: 1100px) { .compose-layout { grid-template-columns: 1fr; } }
+  .preview-panel { position: sticky; top: 1rem; }
+  .preview-label { font-size: 0.75rem; font-weight: 600; color: #64748B; text-transform: uppercase; letter-spacing: 0.05em; margin-bottom: 0.5rem; }
+  #preview-pane { min-height: 400px; background: #F1F5F9; border-radius: 8px; }
+  #preview-pane.loading { opacity: 0.5; }
+</style>
+{% endblock %}
+
 {% block admin_content %}
   <header class="mb-6">
     <a href="{{ url_for('admin.emails') }}" class="text-sm text-slate">&larr; Sent Log</a>
     <h1 class="text-2xl mt-1">Compose Email</h1>
   </header>
 
-  <div class="card" style="padding:1.5rem;max-width:640px">
-    <form method="post" action="{{ url_for('admin.email_compose') }}">
+  <div class="compose-layout">
+    {# ── Left: form ────────────────────────────────────── #}
+    <div>
+      <div class="card" style="padding:1.5rem;">
+        <form id="compose-form" method="post" action="{{ url_for('admin.email_compose') }}">
           <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
 
           <div class="mb-4">
@@ -33,12 +47,28 @@
 
           <div class="mb-4">
             <label class="text-xs font-semibold text-slate block mb-1">Body</label>
-        <textarea name="body" rows="12" class="form-input" placeholder="Plain text (line breaks become &lt;br&gt;)" required>{{ data.get('body', '') }}</textarea>
+            <textarea
+              name="body" rows="14" class="form-input"
+              placeholder="Plain text (line breaks become &lt;br&gt;)"
+              required
+              hx-post="{{ url_for('admin.compose_preview') }}"
+              hx-trigger="input delay:500ms, change"
+              hx-target="#preview-pane"
+              hx-include="#compose-form [name='wrap']"
+              hx-indicator="#preview-pane"
+            >{{ data.get('body', '') }}</textarea>
           </div>
 
           <div class="mb-6">
             <label class="flex items-center gap-2 text-sm">
-          <input type="checkbox" name="wrap" value="1" checked>
+              <input
+                type="checkbox" name="wrap" value="1"
+                {% if data.get('wrap', True) %}checked{% endif %}
+                hx-post="{{ url_for('admin.compose_preview') }}"
+                hx-trigger="change"
+                hx-target="#preview-pane"
+                hx-include="#compose-form textarea[name='body']"
+              >
               Wrap in branded email template
             </label>
           </div>
@@ -49,4 +79,14 @@
           </div>
         </form>
       </div>
+    </div>
+
+    {# ── Right: live preview panel ─────────────────────── #}
+    <div class="preview-panel">
+      <div class="preview-label">Live preview</div>
+      <div id="preview-pane">
+        <p style="padding:2rem;font-size:0.875rem;color:#94A3B8;text-align:center;">Start typing to see a preview…</p>
+      </div>
+    </div>
+  </div>
 {% endblock %}

web/src/padelnomics/admin/templates/admin/partials/email_preview_frame.html

@@ -0,0 +1,8 @@
+{# HTMX partial: sandboxed iframe showing a rendered email preview.
+   Rendered by POST /admin/emails/compose/preview. #}
+<iframe
+  srcdoc="{{ rendered_html | e }}"
+  style="width:100%;height:600px;border:none;border-radius:8px;background:#F1F5F9;"
+  sandbox="allow-same-origin"
+  title="Email preview"
+></iframe>

web/src/padelnomics/templates/emails/admin_compose.html

@@ -0,0 +1,5 @@
+{% extends "emails/_base.html" %}
+
+{% block body %}
+{{ body_html | safe }}
+{% endblock %}