refactor: 项目详情页预览框改为条件渲染，无演示内容时不显示占位符

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

src/components/project/ProjectDetail.tsx

@@ -108,24 +108,17 @@ export async function ProjectDetail({ project, locale }: ProjectDetailProps) {
           ))}
         </div>
 
-        {/* Featured Image/Video Placeholder */}
-        <div className="relative w-full aspect-video border-2 border-black dark:border-gray-600 bg-gray-100 dark:bg-gray-800 mb-10 overflow-hidden shadow-brutal dark:shadow-brutal-dark flex items-center justify-center">
-          {project.source ? (
+        {/* Featured Image/Video Placeholder - only show if source exists */}
+        {project.source && (
+          <div className="relative w-full aspect-video border-2 border-black dark:border-gray-600 bg-gray-100 dark:bg-gray-800 mb-10 overflow-hidden shadow-brutal dark:shadow-brutal-dark">
             <iframe
               src={project.source}
               className="w-full h-full"
               title={`${displayName} demo`}
               allowFullScreen
             />
-          ) : (
-            <div className="text-center">
-              <span className="material-icons text-6xl text-gray-400 dark:text-gray-600 mb-2">
-                smart_toy
-              </span>
-              <p className="text-gray-500 dark:text-gray-400 font-display text-sm">{t('projectPreview')}</p>
-            </div>
-          )}
-        </div>
+          </div>
+        )}
       </header>
 
       {/* Article Content */}