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/plugin.go

@@ -10,8 +10,9 @@ import (
 )
 
 var (
-	_ pact.HasConfig = (*Plugin)(nil)
-	_ pact.HasLang   = (*Plugin)(nil)
+	_ pact.HasConfig        = (*Plugin)(nil)
+	_ pact.HasLang          = (*Plugin)(nil)
+	_ pact.HasMailTemplates = (*Plugin)(nil)
 )
 
 //go:embed config
@@ -20,6 +21,9 @@ var configFS embed.FS
 //go:embed lang
 var langFS embed.FS
 
+//go:embed views/mail
+var mailFS embed.FS
+
 // Plugin is the golem15.hello plugin.
 type Plugin struct{}
 
@@ -32,6 +36,19 @@ func (p *Plugin) Boot(*backpack.App) error     { return nil }
 func (p *Plugin) ConfigFS() fs.FS { return configFS }
 func (p *Plugin) LangFS() fs.FS   { return langFS }
 
+func (p *Plugin) MailTemplatesFS() fs.FS { return mailFS }
+func (p *Plugin) MailTemplates() []string {
+	return []string{
+		"golem15.hello::mail.hello",
+		"golem15.hello::mail.hello-en",
+	}
+}
+func (p *Plugin) MailLayouts() map[string]string {
+	return map[string]string{
+		"hello": "golem15.hello::mail.layouts.hello",
+	}
+}
+
 func init() {
 	party.Register(&Plugin{})
 }