docs: spell out .spbk and .spde extensions

SelfPost backup / SelfPost domain export — in the guide, security notes,
architecture, and Backup/Export panel copy.

Co-Authored-By: Composer <noreply@cursor.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/templates/domain_detail.html

@@ -299,7 +299,7 @@
   <p class="muted"><strong>The export file is a secret</strong> — it contains the
   private DKIM key and application passwords. Transfer it securely and delete it
   after the import, or encrypt it below and move a <code>.spde</code> file
-  instead; the import form asks for the password.</p>
+  (SelfPost domain export) instead; the import form asks for the password.</p>
   {{if .ExportErr}}<p class="error">{{.ExportErr}}</p>{{end}}
   <form method="post" action="/domains/{{.Domain.ID}}/export">
     {{template "encryptfields" .}}