Fix scroll-to-bottom in builder page

- Simplified scrollChatToBottom() with more reliable techniques
- Added scrollIntoView on last message as fallback
- Added additional scroll with direct scrollTop assignment after delay
- Added CSS optimizations for scrolling behavior (-webkit-overflow-scrolling, transform, will-change)
- Added more robust scroll attempts in renderMessages after DOM updates

chat/public/builder.html

@@ -55,6 +55,10 @@
       flex: 1;
       overflow-y: auto;
       min-height: 0;
+      -webkit-overflow-scrolling: touch;
+      scroll-behavior: auto;
+      transform: translateZ(0);
+      will-change: scroll-position;
     }
 
     .app-shell.builder-single>* {