fix(packets): preserve scroll position on filter change + group expand/collapse (#431)

Save scrollTop from #pktLeft before re-render, restore after DOM rebuild.
Covers both the empty-results path (tbody.innerHTML) and the virtual-scroll
path (renderVisibleRows).

Fix lines: packets.js ~1748-1749 (save), ~1821 (restore empty path),
~1840 (restore normal path).

public/packets.js

@@ -1743,6 +1743,10 @@
     const tbody = document.getElementById('pktBody');
     if (!tbody) return;
 
+    // Preserve scroll position across re-render (#431)
+    const scrollContainer = document.getElementById('pktLeft');
+    const savedScrollTop = scrollContainer ? scrollContainer.scrollTop : 0;
+
     // Update dynamic parts of the header
     const countEl = document.querySelector('#pktLeft .count');
     const groupBtn = document.getElementById('fGroup');
@@ -1812,6 +1816,8 @@
       detachVScrollListener();
       const colCount = _getColCount();
       tbody.innerHTML = '<tr><td colspan="' + colCount + '" class="text-center text-muted" style="padding:24px">' + (filters.myNodes ? 'No packets from your claimed/favorited nodes' : 'No packets found') + '</td></tr>';
+      // Restore scroll position after DOM rebuild (#431)
+      if (scrollContainer) scrollContainer.scrollTop = savedScrollTop;
       return;
     }
 
@@ -1829,6 +1835,9 @@
 
     attachVScrollListener();
     renderVisibleRows();
+
+    // Restore scroll position after re-render (#431)
+    if (scrollContainer) scrollContainer.scrollTop = savedScrollTop;
   }
 
   function getDetailPreview(decoded) {