fix(submit): enable optional topic selection by default

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/admin/submit-settings-reference-contract.test.cjs

@@ -103,7 +103,7 @@ test('PHP submit section 字段键与参考主题一致且顺序相同', () => {
 });
 
 test('PHP submit section 包含专题开关与可选专题', () => {
-  assert.match(submitPhp, /'post_article_topics_s',\s*'label'\s*=>\s*'允许选择专题',\s*'type'\s*=>\s*'switch',\s*'default'\s*=>\s*false/);
+  assert.match(submitPhp, /'post_article_topics_s',\s*'label'\s*=>\s*'允许选择专题',\s*'type'\s*=>\s*'switch',\s*'default'\s*=>\s*true/);
   assert.match(submitPhp, /'post_article_topics',\s*'label'\s*=>\s*'投稿允许选择的专题',\s*'type'\s*=>\s*'taxonomy_select',\s*'taxonomy'\s*=>\s*'topics'/);
 });