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/utils/i18nHelpers.test.js

@@ -66,9 +66,6 @@ describe('translate() with real en.json locale', () => {
     it('resolves delete-model modal keys with interpolation', () => {
         installWindowI18n();
 
-        expect(translate('modals.deleteModel.permanentWarning')).toBe(
-            'This will permanently delete the file from disk.',
-        );
         expect(translate('modals.deleteModel.recoverableWarning')).toBe(
             'This will permanently delete the file after 30 seconds unless you undo.',
         );
@@ -77,14 +74,6 @@ describe('translate() with real en.json locale', () => {
         );
     });
 
-    it('resolves the delete-undo settings label', () => {
-        installWindowI18n();
-
-        expect(translate('settings.deleteUndoEnabled')).toBe(
-            'Keep deleted items recoverable for 30 seconds (undo)',
-        );
-    });
-
     it('returns the raw key when no translation exists (fallback contract)', () => {
         installWindowI18n();
         const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});