feat(ui): style app menus and extract glossary dialog

- Add AppMenu and AppMenuItem so menu labels, mnemonics and shortcuts render consistently under org.kde.union
- Move the translation glossary into GlossaryDialog with add, remove and clear-all actions
- Regroup the translation preset and instructions settings layout

src/qml/Header.qml

@@ -79,8 +79,8 @@ ToolBar {
                 enabled: true
 
                 MenuSeparator {}
-                MenuItem { action: root.actions.translateSubtitle }
-                MenuItem { action: root.actions.addSubtitle }
+                AppMenuItem { action: root.actions.translateSubtitle }
+                AppMenuItem { action: root.actions.addSubtitle }
             }
         }