fix: skip overscanned items when computing firstVisibleTimestamp

getVirtualItems() includes overscan rows above the viewport. Filter
by scrollOffset so the minimap indicator tracks the first actually
visible item, not an overscanned one above the fold.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wesm

frontend/src/lib/components/content/MessageList.svelte

@@ -128,9 +128,13 @@
   }
 
   function publishVisibleTimestamp() {
-    const items =
-      virtualizer.instance?.getVirtualItems() ?? [];
+    const v = virtualizer.instance;
+    if (!v) return;
+    const items = v.getVirtualItems();
+    // Skip overscanned items above the viewport.
+    const scrollTop = v.scrollOffset ?? 0;
     for (const vi of items) {
+      if (vi.start < scrollTop) continue;
       const item =
         displayItemsAsc[
           ui.sortNewestFirst