Render inbound email HTML body in admin inbox

Worker now extracts both text/plain and text/html MIME parts and sends
htmlBody in the webhook payload. Server stores it on the submission.
Admin inbox renders htmlBody in a sandboxed iframe when present, falling
back to plain text with whitespace preserved.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AdminPage.tsx

@@ -628,6 +628,7 @@ interface ContactSubmission {
   archived?: boolean
   source?: 'contact-form' | 'download' | 'inbound-email'
   inboundTo?: string
+  htmlBody?: string | null
 }
 
 interface ContactReplyDraft {
@@ -5051,7 +5052,20 @@ export default function AdminPage({ content, onSave, onLogout }: Props) {
                           </div>
 
                           <div className="admin-email-body">
-                            <p>{selected.message}</p>
+                            {selected.htmlBody ? (
+                              <iframe
+                                srcDoc={selected.htmlBody}
+                                sandbox="allow-same-origin"
+                                style={{ width: '100%', minHeight: '320px', border: 'none', background: '#fff', borderRadius: '4px' }}
+                                onLoad={e => {
+                                  const iframe = e.currentTarget
+                                  const h = iframe.contentDocument?.documentElement?.scrollHeight
+                                  if (h) iframe.style.height = `${h}px`
+                                }}
+                              />
+                            ) : (
+                              <p style={{ whiteSpace: 'pre-wrap' }}>{selected.message}</p>
+                            )}
                           </div>
 
                           <div className="admin-actions admin-actions--maintenance">