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/hello_test.go

@@ -16,6 +16,7 @@ import (
 	"git.golem15.com/golem15/summercms/pact"
 	"git.golem15.com/golem15/summercms/party"
 	"git.golem15.com/golem15/summercms/phrasebook"
+	"git.golem15.com/golem15/summercms/postcard"
 	"git.golem15.com/golem15/summercms/surf"
 )
 
@@ -142,6 +143,35 @@ func TestHelloPluginResolvesLang(t *testing.T) {
 	}
 }
 
+func TestHelloPluginSendsMail(t *testing.T) {
+	cfg, err := compass.Load("config")
+	if err != nil {
+		t.Fatal(err)
+	}
+	app := backpack.New(cfg)
+	if _, err := party.Activate(app, []string{"golem15.hello"}); err != nil {
+		t.Fatal(err)
+	}
+	mail, ok := app.Lookup[postcard.Mailer]()
+	if !ok {
+		t.Fatal("mailer not published")
+	}
+	if err := mail.Send(t.Context(), postcard.Message{
+		Template: "golem15.hello::mail.hello",
+		To:       []string{"ada@example.test"},
+		Vars:     map[string]any{"Name": "Ada", "URL": "https://example.test"},
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := mail.Send(t.Context(), postcard.Message{
+		Template: "golem15.hello::mail.hello-en",
+		To:       []string{"ada@example.test"},
+		Vars:     map[string]any{"Name": "Ada", "URL": "https://example.test"},
+	}); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestTypedItemRoute(t *testing.T) {
 	cfg, err := compass.Load("config")
 	if err != nil {