fix: measure VSCROLL_ROW_HEIGHT and theadHeight dynamically (#625)

## Summary

Replaces hardcoded `VSCROLL_ROW_HEIGHT = 36` and `theadHeight = 40` in
the virtual scroll logic with dynamic DOM measurement, so the values
stay correct if CSS changes.

## Changes

- `VSCROLL_ROW_HEIGHT`: measured once from the first rendered data row's
`offsetHeight` after the initial full rebuild. Falls back to 36px until
measurement occurs.
- `theadHeight`: measured from the actual `<thead>` element's
`offsetHeight` on every `renderVisibleRows` call. Falls back to 40px if
no thead is found.
- Both variables are now `let` instead of `const` to allow runtime
updates.

## Performance

No performance impact — both measurements are single `offsetHeight`
reads (no reflow triggered since the DOM was just written). Row height
measurement runs only once (guarded by `_vscrollRowHeightMeasured`
flag). Thead measurement is a single property read per scroll event.

Fixes #407

Co-authored-by: you <you@example.com>

public/packets.js

@@ -63,7 +63,9 @@
   const getParsedDecoded = window.getParsedDecoded;
 
   // --- Virtual scroll state ---
-  const VSCROLL_ROW_HEIGHT = 36;  // estimated row height in px
+  let VSCROLL_ROW_HEIGHT = 36;    // measured dynamically on first render; fallback 36px
+  let _vscrollRowHeightMeasured = false;
+  let _vscrollTheadHeight = 40;   // measured dynamically on first render; fallback 40px
   const VSCROLL_BUFFER = 30;      // extra rows above/below viewport
   let _displayPackets = [];       // filtered packets for current view
   let _displayGrouped = false;    // whether _displayPackets is in grouped mode
@@ -1277,8 +1279,10 @@
     // Calculate visible range based on scroll position
     const scrollTop = scrollContainer.scrollTop;
     const viewportHeight = scrollContainer.clientHeight;
-    // Account for thead height (~40px)
-    const theadHeight = 40;
+    // Account for thead height (measured dynamically)
+    const theadEl = scrollContainer.querySelector('thead');
+    if (theadEl) _vscrollTheadHeight = theadEl.offsetHeight || _vscrollTheadHeight;
+    const theadHeight = _vscrollTheadHeight;
     const adjustedScrollTop = Math.max(0, scrollTop - theadHeight);
 
     // Find the first entry whose cumulative row offset covers the scroll position
@@ -1336,6 +1340,14 @@
       tbody.appendChild(topSpacer);
       tbody.insertAdjacentHTML('beforeend', visibleHtml);
       tbody.appendChild(bottomSpacer);
+      // Measure actual row height from first rendered data row (#407)
+      if (!_vscrollRowHeightMeasured) {
+        const firstRow = topSpacer.nextElementSibling;
+        if (firstRow && firstRow !== bottomSpacer) {
+          const h = firstRow.offsetHeight;
+          if (h > 0) { VSCROLL_ROW_HEIGHT = h; _vscrollRowHeightMeasured = true; }
+        }
+      }
       if (window.__PERF_LOG_RENDER) console.log('[perf] renderVisibleRows: full rebuild %d entries, %.2fms', endIdx - startIdx, performance.now() - _rvr_t0);
       return;
     }