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/src/pages/user/sop/index.vue

@@ -993,6 +993,7 @@ const sendQuestion = async (
     }
     retireEarlierMemoryCandidate(nextResult.memoryCandidate?.id);
     const spoken = (nextResult.answer || '').trim();
+    const shouldPreloadVoice = Boolean(spoken) && !isNativeAppRuntime();
     replaceMsg(pending.id, {
       id: pending.id,
       role: 'master',
@@ -1004,12 +1005,12 @@ const sendQuestion = async (
       feedbackReasonCodes: [],
       feedbackComment: '',
       citationsOpen: false,
-      voiceStatus: spoken ? 'preparing' : 'unavailable',
+      voiceStatus: shouldPreloadVoice ? 'preparing' : 'unavailable',
       voiceDuration: null,
       unread: Boolean(spoken),
       transcriptOpen: false
     });
-    if (spoken) void prepareVoice(pending.id, spoken);
+    if (shouldPreloadVoice) void prepareVoice(pending.id, spoken);
     if (!nextResult.answer && !nextResult.snippets?.length && !nextResult.memoryCandidate) {
       pushSystem('未检索到可引用内容，换个问法试试');
     }