fix: stack modal waveform charts vertically to match live events view

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sfm/sfm_webapp.html

@@ -1577,6 +1577,8 @@ function _buildWaveformCharts(data, chartsEl, emptyEl, chartsStore) {
   const times = Array.from({length: decoded}, (_, i) => ((i - pretrig) / sr * 1000).toFixed(2));
   if (emptyEl) emptyEl.style.display = 'none';
   chartsEl.style.display = 'flex';
+  chartsEl.style.flexDirection = 'column';
+  chartsEl.style.gap = '8px';
   chartsEl.innerHTML = '';
 
   const micPeakPsi = data.peak_values?.micl_psi ?? null;