feat(ui): finished turns keep their tool tree, reasoning and say bubbles, the thread stops dragging the reader down

ui/src/lib/panel/say-bubble.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+  import MessageSquareIcon from "@lucide/svelte/icons/message-square";
+  import Markdown from "./markdown.svelte";
+
+  let { text }: { text: string } = $props();
+</script>
+
+<div class="mx-1 flex gap-2 rounded-md bg-primary/8 px-2.5 py-1.5">
+  <MessageSquareIcon class="mt-1 size-3.5 shrink-0 text-link" />
+  <Markdown class="min-w-0 flex-1" {text} />
+</div>