feat: add placeholder height for topic content area

Add minimum height placeholder for NewsContentView while content is rendering:
- Use full screen height as initial placeholder to hide reply section
- Shrink to actual content height when rendering completes
- Smooth animation transition using onRenderCompleted callback

This prevents layout jumps and hides reply section until content is ready.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: graycreate

V2er/View/FeedDetail/NewsContentView.swift

@@ -17,6 +17,7 @@ struct NewsContentView: View {
     @State private var navigateToNode: String? = nil
     @State private var navigateToBrowserURL: URL? = nil
     @State private var navigateToSafariURL: URL? = nil
+    @State private var contentReady = false
 
     private var useBuiltinBrowser: Bool {
         store.appState.settingState.useBuiltinBrowser
@@ -39,8 +40,14 @@ struct NewsContentView: View {
                     // Open image in SafariView for now
                     openInSafari(url)
                 }
+                .onRenderCompleted { _ in
+                    withAnimation(.easeInOut(duration: 0.2)) {
+                        contentReady = true
+                    }
+                }
                 .padding(.horizontal, 12)
                 .padding(.vertical, 8)
+                .frame(minHeight: contentReady ? 0 : UIScreen.main.bounds.height)
 
             Divider()
         }