feat(19.6.1-01): Administrators screen and cross-controller record actions

- pact.HasAdminActionExtensions / AdminActionExtension: plugins add record
  actions to any admin controller by ID, merged at boot before permission
  validation (unknown target, no preview, duplicate name stop the boot)
- internal summercms.cabana pseudo-plugin, appended only when some plugin
  registers admin controllers: Administrators list and preview, settings
  link under System, backend.manage_users and backend.manage_roles with no
  role grants
- backend::lang user, user.role and permissions copy (en, pl)
- smoke tests: compile-time seam and settings, Postgres list/preview/action

modules/pact/capabilities.go

@@ -375,6 +375,38 @@ type HasAdminRecordActions interface {
 	AdminRecordActions() []AdminRecordAction
 }
 
+// AdminActionExtension adds actions to an admin controller that another
+// plugin owns, or that the plugin itself owns. Controller is the controller
+// ID (vendor.plugin.controller) of any activated plugin's admin controller,
+// the admin framework's own controllers included.
+//
+// RecordActions are offered on the target controller's preview screen after
+// the owner's record actions, in plugin activation order. Each one keeps its
+// own Permissions, Applies, Confirm and Danger and runs exactly like the
+// owner's record actions: the framework loads and row-locks the record
+// through the target's FormExtendQuery scope inside the action's transaction
+// and hands it to Run, which reads that transaction with
+// cabana.TxFromContext. An administrator needs the target controller's
+// RequiredPermissions and the action's own Permissions. A record action
+// carries no input beyond the record.
+//
+// The admin compiles extensions at boot, only when it is active (some
+// plugin registers admin controllers). An unknown controller ID, a target
+// without a preview block, a name the owner or an earlier extension already
+// registers on that controller, a reserved name (create, delete), a name that
+// is not an identifier, a missing Run or an empty Label stops the boot. Only
+// record actions are carried for now; the struct leaves room for more kinds.
+type AdminActionExtension struct {
+	Controller    string
+	RecordActions []AdminRecordAction
+}
+
+// HasAdminActionExtensions is implemented by plugins (not admin controllers)
+// that add actions to admin controllers by controller ID.
+type HasAdminActionExtensions interface {
+	AdminActionExtensions() []AdminActionExtension
+}
+
 // AdminPartialData supplies the view model a controller partial template
 // renders (config_list.yaml headerPartial, fields.yaml `type: partial`). name
 // is the partial name; record is the scoped record for a form partial on an