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>

examples/hello/plugins/base/lang/en/lang.yaml

@@ -0,0 +1,3 @@
+greeting: Hello
+labels:
+  pluginName: Hello plugin

examples/hello/plugins/base/lang/pl/lang.yaml

@@ -0,0 +1,3 @@
+greeting: Cześć
+labels:
+  pluginName: Wtyczka Hello

examples/hello/plugins/base/plugin.go

@@ -9,11 +9,17 @@ import (
 	"git.golem15.com/golem15/summercms/party"
 )
 
-var _ pact.HasConfig = (*Plugin)(nil)
+var (
+	_ pact.HasConfig = (*Plugin)(nil)
+	_ pact.HasLang   = (*Plugin)(nil)
+)
 
 //go:embed config
 var configFS embed.FS
 
+//go:embed lang
+var langFS embed.FS
+
 // Plugin is the golem15.hello plugin.
 type Plugin struct{}
 
@@ -24,6 +30,7 @@ func (p *Plugin) Register(*backpack.App) error { return nil }
 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 init() {
 	party.Register(&Plugin{})