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 }}**.