feat(04-03): register plugin mail layouts and publish the mailer at boot

- Map short layout names onto dotted layout files and wrap text/HTML
- party.Activate publishes postcard.Mailer before Boot and names missing files
- Hello plugin sends through the app-scoped mailer

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

examples/hello/plugins/base/views/mail/hello-en.htm

@@ -1,6 +1,6 @@
 subject = "Hello {{ .Name }}"
 description = "English hello greeting"
-layout = "default"
+layout = "hello"
 ==
 Hello **{{ .Name }}**.
 

examples/hello/plugins/base/views/mail/hello.htm

@@ -1,6 +1,6 @@
 subject = "Witaj {{ .Name }}"
 description = "Hello greeting"
-layout = "default"
+layout = "hello"
 ==
 Witaj **{{ .Name }}**.
 

examples/hello/plugins/base/views/mail/layouts/hello.htm

@@ -0,0 +1,14 @@
+name = "Hello"
+==
+-- {{ .Content }} --
+==
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+<style type="text/css">{{ .brandCss }}{{ .css }}</style>
+</head>
+<body>
+<div class="layout-hello">{{ .Content }}</div>
+</body>
+</html>