fix(mobile): stop preloading answer TTS on native App

On native App the eager TTS request delayed the answer bubble: the message was
created in `preparing` state, so the text stayed behind a voice line that was
still waiting on a round trip.

Skip the preload when `isNativeAppRuntime()`, marking the voice `unavailable`
so the text and citations render immediately. The existing `unavailable`
branch already exposes a play button that calls `toggleAnswerSpeech`, so voice
stays reachable on demand instead of being dropped. H5 keeps preloading.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

mobile-uni/tests/agent.test.mjs

@@ -38,6 +38,14 @@ test('Agent 响应按结构化状态渲染来源、澄清和确认动作', async
   assert.doesNotMatch(page, /result\.answer\.(?:includes|match|startsWith)/);
 });
 
+test('原生 App 立即显示 Agent 文字回答，语音仅按需生成', async () => {
+  const page = await source('../src/pages/user/sop/index.vue');
+
+  assert.match(page, /const shouldPreloadVoice = Boolean\(spoken\) && !isNativeAppRuntime\(\)/);
+  assert.match(page, /voiceStatus: shouldPreloadVoice \? 'preparing' : 'unavailable'/);
+  assert.match(page, /if \(shouldPreloadVoice\) void prepareVoice\(pending\.id, spoken\)/);
+});
+
 test('Agent 确认卡通过不透明 draftId 调用动作端点', async () => {
   const [assistantService, card] = await Promise.all([
     source('../src/services/personal-assistant.ts'),