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>

postcard/mailer.go

@@ -3,6 +3,10 @@ package postcard
 import (
 	"context"
 	"fmt"
+	"strings"
+
+	"git.golem15.com/golem15/summercms/backpack"
+	"git.golem15.com/golem15/summercms/pact"
 )
 
 // Message is the Send contract for plugin callers.
@@ -70,3 +74,44 @@ func (m *mailer) Send(ctx context.Context, msg Message) error {
 	}
 	return nil
 }
+
+// Activate publishes an app-scoped Mailer before plugin Boot. Catalog files
+// are validated later at each plugin's Boot transition.
+func Activate[P interface{ ID() string }](app *backpack.App, _ []P) error {
+	if app == nil {
+		return fmt.Errorf("postcard: app is nil")
+	}
+	opts := Options{}
+	if app.Config != nil {
+		opts.From = strings.TrimSpace(app.Config.String("mail.from"))
+		opts.CSS = app.Config.String("mail.css")
+		opts.BrandCSS = app.Config.String("mail.brandCss")
+	}
+	m := &mailer{
+		catalog: NewCatalog(),
+		driver:  NewMemoryDriver(),
+		opts:    opts,
+	}
+	if err := app.Publish[Mailer](m); err != nil {
+		return fmt.Errorf("postcard: %w", err)
+	}
+	return nil
+}
+
+// BootPlugin registers a plugin's declared mail templates and layouts.
+// Missing files and unknown layout aliases fail with the dotted name.
+func BootPlugin[P interface{ ID() string }](app *backpack.App, p P) error {
+	hm, ok := any(p).(pact.HasMailTemplates)
+	if !ok {
+		return nil
+	}
+	got, ok := app.Lookup[Mailer]()
+	if !ok {
+		return fmt.Errorf("postcard: mailer not published")
+	}
+	m, ok := got.(*mailer)
+	if !ok || m == nil || m.catalog == nil {
+		return fmt.Errorf("postcard: mailer not published")
+	}
+	return m.catalog.Register(p.ID(), hm.MailTemplatesFS(), hm.MailTemplates(), hm.MailLayouts())
+}