fix(panel): column-reverse thread keeps the bottom, no island in the plugin

ui/src/lib/panel/chat-view.svelte

@@ -79,20 +79,17 @@
     }
   }
 
+  // The scroller is column-reverse: its origin is the bottom, scrollTop is
+  // zero there and goes negative upward, and the browser keeps the view at
+  // the bottom while content grows - no matter how late markdown lands.
   function nearBottom(): boolean {
-    if (!scroller) {
-      return true;
-    }
-    const left =
-      scroller.scrollHeight - scroller.scrollTop - scroller.clientHeight;
-    return left < NEAR_BOTTOM_PX;
+    return !scroller || Math.abs(scroller.scrollTop) < NEAR_BOTTOM_PX;
   }
 
   function toBottom() {
     if (scroller) {
-      scroller.scrollTop = scroller.scrollHeight;
+      scroller.scrollTop = 0;
     }
-    // Whatever actually scrolls - this box, or a host's pane around it.
     end?.scrollIntoView({ block: "end" });
   }
 
@@ -211,7 +208,7 @@
 </script>
 
 <div
-  class="min-h-0 flex-1 overflow-y-auto overflow-x-hidden px-3 py-3 @md:px-6"
+  class="flex min-h-0 flex-1 flex-col-reverse overflow-y-auto overflow-x-hidden px-3 py-3 @md:px-6"
   onscroll={() => {
     if (Date.now() > settleUntil) {
       pinned = nearBottom();