feat(submit): add user media library

tests/widget-system/runtime-action-hard-cut-contract.test.cjs

@@ -17,6 +17,8 @@ test('legacy theme action endpoint stays removed from runtime bundles', () => {
     assert.doesNotMatch(source, /_win\.bd\.on\(['"]click['"],\s*['"]\[data-action\]['"]/, file)
     assert.doesNotMatch(source, /\baction_ajax\s*\(/, file)
     assert.doesNotMatch(source, /\/action\/action\.php/, file)
+    assert.doesNotMatch(source, /action=download_file/, `${file} must not restore the legacy download-file handler`)
+    assert.doesNotMatch(source, /download-nonce/, `${file} must not restore the legacy download nonce attribute`)
   }
 })