修正讲解音频时长显示

- 未加载媒体元数据时默认显示零时长
- 使用音频文件真实媒体时长更新播放器
- 避免使用不准确的接口时长覆盖播放器显示
- 补充音频时长回归测试

Made-with: Proma

tests/unit/ExhibitDetailAudioFirst.spec.ts

@@ -164,7 +164,7 @@ describe('讲解详情音频优先布局', () => {
     expect(wrapper.get('.content-section').classes()).toContain('is-english')
     expect(wrapper.find('.section-title').exists()).toBe(false)
     expect(wrapper.get('.section-text').text()).toContain('detailed English narration')
-    expect(wrapper.get('.detail-audio-time').text()).toBe('0:00 / 1:21')
+    expect(wrapper.get('.detail-audio-time').text()).toBe('0:00 / 0:00')
     expect(wrapper.get('.detail-audio-speed').attributes('aria-label')).toBe('播放速度 1.0倍')
   })