refactor(delete): make undo unconditional, remove undo toggle and button delay

- Remove delete_undo_enabled setting (backend default, frontend state,
  settings modal UI, 10 locales); staged deletes with 30s undo are now
  the only delete path and stale settings keys are silently ignored
- Remove the 1500ms delete-button arm delay (armDeleteButton) from all
  delete modals; misclicks are recoverable via the undo toast
- Delete modal always shows the recoverable warning
- Log the first staged file path in staging log lines for easier support

tests/frontend/pages/embeddingsPage.test.js

@@ -26,7 +26,6 @@ vi.mock('../../../static/js/utils/modalUtils.js', () => ({
   closeDeleteModal: closeDeleteModalMock,
   confirmExclude: confirmExcludeMock,
   closeExcludeModal: closeExcludeModalMock,
-  armDeleteButton: vi.fn(),
 }));
 
 vi.mock('../../../static/js/api/apiConfig.js', () => ({