fix(chat): give the sorted Activity block the live timeline's row rhythm

The Activity block stacked its tool rows with an extra 0.375rem gap on top
of each row's own padding, while the live timeline stacks the same rows
with nothing between them. Both views now share the 36px row step.

Testing: measured both render modes on one session in headless Chrome;
ui type-check and lint.

packages/ui/src/components/chat/message/parts/ProgressiveGroup.tsx

@@ -943,7 +943,7 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
     if (!showHeader) {
         return (
             <FadeInOnReveal>
-                <div className="mt-1 mb-2 space-y-1.5">{renderedRows}</div>
+                <div className="mt-1 mb-2">{renderedRows}</div>
             </FadeInOnReveal>
         );
     }
@@ -986,7 +986,10 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
                                 +{previewHiddenCount} more...
                             </button>
                         ) : null}
-                        <div className="space-y-1.5">{renderedRows}</div>
+                        {/* No gap between rows: each row carries its own padding, and
+                            the live timeline stacks the same rows with nothing between
+                            them, so the sorted view keeps the identical rhythm. */}
+                        <div>{renderedRows}</div>
                     </div>
                 ) : null}
             </div>