feat(04-02): load namespaced translations from embedded plugins

- Walk HasLang FS for lang/<locale>/<group>.yaml and flatten nested keys
- Publish one phrasebook.Translator before Boot with en/en defaults
- Hello plugin resolves en/pl vendor.plugin::group.key after Activate

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

party/registry.go

@@ -7,6 +7,7 @@ import (
 
 	"git.golem15.com/golem15/summercms/backpack"
 	"git.golem15.com/golem15/summercms/pact"
+	"git.golem15.com/golem15/summercms/phrasebook"
 )
 
 // Plugin is the required descriptor every compiled plugin implements.
@@ -116,6 +117,9 @@ func activate(registered []Plugin, app *backpack.App, ids []string) ([]Plugin, e
 			return nil, fmt.Errorf("party: register %s: %w", p.ID(), err)
 		}
 	}
+	if err := phrasebook.Activate(app, ordered); err != nil {
+		return nil, fmt.Errorf("party: lang: %w", err)
+	}
 	for _, p := range ordered {
 		if err := p.Boot(app); err != nil {
 			return nil, fmt.Errorf("party: boot %s: %w", p.ID(), err)