feat(17-04): menu plugin admin with registry types, URL checks and tree reorder

- golem15.menu plugin, tables golem15_menu_menus and golem15_menu_menu_entries with the PHP columns plus a nullable url_pattern (D-02)
- MenuEntry BeforeSave ports PHP beforeSave: nest columns 0, menu from the parent, sort_order after the menu's last
- entry type dropdown: url, divider and the signpost registry types; reference depends on type through signpost.ReferenceOptions
- URL scheme allowlist (errors.url_scheme), target _self or _blank, menu and parent cycle checks
- menu filter, toolbar reorder/create/delete and config_reorder.yaml partitioned by menu
- navigation Menus with Menus and Menu entries, permission golem15.menu.manage_menu, en and pl phrasebooks
- TestMenuAdminSmoke

admin.go

@@ -0,0 +1,45 @@
+package menu
+
+import (
+	"embed"
+	"io/fs"
+
+	"git.golem15.com/golem15/sm-menu-plugin/controllers"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/pact"
+)
+
+// adminFS holds the admin YAML file by file: the controllers directory also
+// holds Go sources.
+//
+//go:embed controllers/menus/config_form.yaml controllers/menus/config_list.yaml models/menu/fields.yaml models/menu/columns.yaml
+//go:embed controllers/menuentries/config_form.yaml controllers/menuentries/config_list.yaml controllers/menuentries/config_filter.yaml controllers/menuentries/config_reorder.yaml
+//go:embed models/menuentry/fields.yaml models/menuentry/columns.yaml
+var adminFS embed.FS
+
+//go:embed lang
+var langFS embed.FS
+
+// AdminFS serves the admin YAML.
+func (p *Plugin) AdminFS() fs.FS { return adminFS }
+
+// LangFS is the embedded English and Polish phrasebooks.
+func (p *Plugin) LangFS() fs.FS { return langFS }
+
+// AdminControllers hands the controllers a lazy accessor of the booted app,
+// so they read the database and the menu item type registry at request
+// time.
+func (p *Plugin) AdminControllers() []pact.AdminController {
+	return controllers.AdminControllers(func() *backpack.App {
+		if p == nil {
+			return nil
+		}
+		return p.app
+	})
+}
+
+var (
+	_ pact.AdminAssets         = (*Plugin)(nil)
+	_ pact.HasAdminControllers = (*Plugin)(nil)
+	_ pact.HasLang             = (*Plugin)(nil)
+)