fix: 标题预览仅在Step4显示,Step5不再显示标题叠加层 (#1392)

- showTitlePreview 条件从 currentStep>=4&&<=5 改为 currentStep===4
- Step5预览视频页面只展示预览视频本身,不再叠加标题Canvas
- 避免与右侧面板预览视频重复导致视觉拥挤

apps/web/src/pages/generate/GeneratePage.tsx

@@ -265,7 +265,7 @@ const GeneratePage: React.FC = () => {
               onRegenerate={step5Preview.regeneratePreview}
               titleText={titleSettings.title}
               titleSettings={titleSettings}
-              showTitlePreview={currentStep >= 4 && currentStep <= 5}
+              showTitlePreview={currentStep === 4}
               sourceVideoUrl={sourceVideoUrl}
             />
           )}