style(chat): improve chat layout spacing and empty state styling

- Add top margin (rb:mt-2) to bottom label in ChatContent for better spacing
- Wrap Chat component in centered container with max-width (760px) and top padding
- Replace AnalysisEmptyIcon with BgImg in empty state for improved visual consistency
- Add size prop [320,180] to Empty component for proper empty state dimensions
- Adjust contentClassName spacing for better layout alignment
- Improves overall chat interface visual hierarchy and spacing consistency

web/src/components/Chat/ChatContent.tsx

@@ -68,7 +68,7 @@ const ChatContent: FC<ChatContentProps> = ({
                 </div>
                 {/* 底部标签（如时间戳、用户名等） */}
                 {labelPosition === 'bottom' &&
-                  <div className="rb:text-[#5B6167] rb:text-[12px] rb:leading-4 rb:font-regular">
+                  <div className="rb:text-[#5B6167] rb:text-[12px] rb:leading-4 rb:font-regular rb:mt-2">
                     {labelFormat(item)}
                   </div>
                 }