ui: smart auto-scroll in LogPanel (#530)

Pause auto-scroll when the user scrolls up to review logs, and resume
when they scroll back to the bottom.

- add `userScrolledUp` state variable
- add `handleScroll` to detect scroll position with 40px threshold
- guard the auto-scroll effect with `!userScrolledUp`

closes #529

ui-svelte/src/components/LogPanel.svelte

@@ -65,10 +65,17 @@
   });
 
   let preElement: HTMLPreElement;
+  let userScrolledUp = $state(false);
 
-  // Auto scroll to bottom when logs change
+  function handleScroll() {
+    if (!preElement) return;
+    const { scrollTop, scrollHeight, clientHeight } = preElement;
+    userScrolledUp = scrollHeight - scrollTop - clientHeight > 40;
+  }
+
+  // Auto scroll to bottom when logs change, unless user has scrolled up
   $effect(() => {
-    if (preElement && filteredLogs) {
+    if (preElement && filteredLogs && !userScrolledUp) {
       preElement.scrollTop = preElement.scrollHeight;
     }
   });
@@ -127,6 +134,6 @@
     {/if}
   </div>
   <div class="rounded-lg bg-background font-mono text-sm flex-1 overflow-hidden">
-    <pre bind:this={preElement} class="{textWrapClass} {fontSizeClass} h-full overflow-auto p-4">{filteredLogs}</pre>
+    <pre bind:this={preElement} onscroll={handleScroll} class="{textWrapClass} {fontSizeClass} h-full overflow-auto p-4">{filteredLogs}</pre>
   </div>
 </div>