Split internal/web into subpackages before domain-admin growth.

Lay out view, auth, validate, and handlers under internal/web while keeping
the cmd/panel API unchanged; update roadmap and changelog for web-split closure.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/view/templates/mail_queue_body.html

@@ -0,0 +1,6 @@
+{{define "mail_queue_body"}}
+<div id="mail-queue-body" data-poll hx-get="/mail-queue/body" hx-trigger="load" hx-swap="outerHTML">
+  {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+  <span class="code">{{if .Output}}{{.Output}}{{else}}Queue is empty.{{end}}</span>
+</div>
+{{end}}