feat: add back button to test edit form

TestForm now accepts optional onBack/backLabel props.
TestEdit passes 'К просмотру теста' back button when in edit mode.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/TestEdit/index.tsx

@@ -92,6 +92,8 @@ export default function TestEdit() {
         isPending={isPending}
         submitLabel="Сохранить"
         onCancel={() => setEditMode(false)}
+        onBack={() => setEditMode(false)}
+        backLabel="К просмотру теста"
       />
     )
   }