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.html

@@ -0,0 +1,12 @@
+{{/* A table of queue ids, sizes, senders and recipients: the whole column
+     rather than the reading measure (see the "wide" block in layout.html). */}}
+{{define "wide"}}wide{{end}}
+
+{{define "content"}}
+<h1>Mail queue</h1>
+
+<div class="card">
+  <h2>Pending messages</h2>
+  {{template "mail_queue_body" .}}
+</div>
+{{end}}