viewers: default mic to dB(L) + add Mic-unit toggle (dBL ↔ psi)

The sidecar-modal waveform plot was rendering mic in raw psi, while the
rest of SFM (history table column, peaks block, live-device chart,
event detail modal mic field) had already converted to dB(L) — matching
the BW Event Report convention.  Unifying.

Both viewers now:
  - Default mic chart values + axis title + peak label to dB(L)
  - Provide a header toggle ("Mic: dBL" pill) to flip to psi
  - Persist the preference via localStorage (sfm_mic_unit)
  - Re-render the open chart immediately on toggle

Conversion: dBL = 20 * log10(psi / 2.9e-9), where 2.9e-9 psi is the
20 µPa reference pressure already defined for the rest of the webapp.
Non-positive psi samples (log undefined) render as null; Chart.js
handles them as gaps in line mode and missing bars in histogram mode.

Also fixes event_browser.html's stats table — the MicL row was
hard-coding "<value> psi"; now honors the same toggle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sfm/sfm_webapp.html

@@ -818,6 +818,12 @@
     <span class="ft-dot"></span>
     <span>Force refresh</span>
   </label>
+  <div class="hdr-sep"></div>
+  <button id="mic-unit-toggle" class="section-btn"
+          onclick="_setMicUnit(_getMicUnit() === 'dBL' ? 'psi' : 'dBL')"
+          title="Toggle microphone display unit (dBL ↔ psi) for waveform plots.  Affects all mic charts; persists across page loads.">
+    Mic: dBL
+  </button>
 </header>
 
 <!-- ════════════════════════════════════════════════════════════════
@@ -2560,6 +2566,29 @@ const _SC_CHANNEL_COLORS = {
 const _SC_CHANNEL_ORDER = ['MicL', 'Long', 'Vert', 'Tran'];
 let _scCharts = {};
 
+// User preference for how mic is displayed in plots — dBL (default,
+// matches BW printout convention + the rest of SFM) or psi (the raw
+// sample unit).  Toggleable via the header pill; persists in localStorage.
+function _getMicUnit() {
+  return localStorage.getItem('sfm_mic_unit') === 'psi' ? 'psi' : 'dBL';
+}
+function _setMicUnit(u) {
+  localStorage.setItem('sfm_mic_unit', u === 'psi' ? 'psi' : 'dBL');
+  _refreshMicUnitToggleLabel();
+  // Re-render the open modal so the change is immediately visible.
+  if (_scCurrentEventId) openSidecarModal(_scCurrentEventId);
+}
+function _refreshMicUnitToggleLabel() {
+  const b = document.getElementById('mic-unit-toggle');
+  if (b) b.textContent = `Mic: ${_getMicUnit()}`;
+}
+// Convert a psi value to dB(L).  Returns null for non-positive values
+// (log of zero is undefined) — Chart.js handles null as a gap in the line.
+function _psiToDbl(psi) {
+  if (psi == null || !(psi > 0)) return null;
+  return 20 * Math.log10(psi / DBL_REF);
+}
+
 // Adaptive decimal formatter — scientific notation is reserved for truly
 // extreme values (10000+ or sub-0.0001).  Normal-range values (most peaks
 // fall here) render as decimals with sensible precision.  Replaces the
@@ -2610,17 +2639,30 @@ function _renderScWaveform(data) {
   );
   const lastCh = withData[withData.length - 1];
 
+  const micUnit = _getMicUnit();   // user preference: 'dBL' or 'psi'
+
   for (const ch of _SC_CHANNEL_ORDER) {
     const chData = channels[ch];
     if (!chData) continue;
-    const values = chData.values || [];
+    let values = chData.values || [];
+    let chUnit = chData.unit || '';
+    let chPeak = chData.peak;
+
+    // Mic channel: convert from raw psi to dB(L) when user prefers dBL
+    // (default).  Mic samples that are zero/negative become null (Chart.js
+    // renders them as gaps in line mode, zero-height bars in histogram mode).
+    if (ch === 'MicL' && chUnit === 'psi' && micUnit === 'dBL') {
+      values = values.map(_psiToDbl);
+      chPeak = _psiToDbl(chPeak);
+      chUnit = 'dB(L)';
+    }
 
     const wrap = document.createElement('div');
     wrap.style.cssText = 'background:var(--surface);border:1px solid var(--border2);border-radius:6px;padding:6px 30px 4px 10px';
     const lbl = document.createElement('div');
     lbl.style.cssText = `font-size:10px;font-weight:600;letter-spacing:0.05em;text-transform:uppercase;margin-bottom:2px;color:${_SC_CHANNEL_COLORS[ch]};display:flex;justify-content:space-between`;
-    const peakStr = chData.peak != null
-      ? `peak ${_fmtPeak(chData.peak, chData.unit)}`
+    const peakStr = chPeak != null
+      ? `peak ${_fmtPeak(chPeak, chUnit)}`
       : '';
     lbl.innerHTML = `<span>${ch}</span><span style="color:var(--text-dim);font-weight:normal">${peakStr}</span>`;
     wrap.appendChild(lbl);
@@ -2716,7 +2758,7 @@ function _renderScWaveform(data) {
               title: items => isHistogram
                 ? `interval ${items[0].label}`
                 : `t = ${items[0].label} ms`,
-              label: item => `${ch}: ${_fmtPeak(item.raw, chData.unit)}`,
+              label: item => `${ch}: ${_fmtPeak(item.raw, chUnit)}`,
             },
           },
         },
@@ -2730,7 +2772,7 @@ function _renderScWaveform(data) {
             ...yBounds,
             ticks: { color: '#484f58', maxTicksLimit: 4 },
             grid:  { color: '#21262d' },
-            title: { display: true, text: chData.unit || '', color: '#484f58', font: { size: 9 } },
+            title: { display: true, text: chUnit, color: '#484f58', font: { size: 9 } },
           },
         },
       },
@@ -3058,6 +3100,9 @@ document.addEventListener('keydown', e => {
 // hit localhost:8200, 10.0.0.44:8200, or anything else.
 document.getElementById('api-base').value = window.location.origin;
 
+// Reflect any persisted mic-unit preference in the header pill on load
+_refreshMicUnitToggleLabel();
+
 // We default to Database view → trigger initial history + units load
 // (switchSection handles this when clicked, but we never click on first paint).
 if (currentSection === 'db') {