viewers: symmetric Y-axis on geo waveforms + clarify timestamp labels

Two fixes from the second screenshot review:

1. Geophone waveform Y-axis now renders SYMMETRIC around zero — zero
   line sits in the middle of the chart, signal goes both above and
   below.  Standard seismograph display convention; matches the
   Instantel printout look.  Previously Chart.js auto-scaled to the
   data range so e.g. Vert showing values from -0.005 to -0.015 had
   the zero line completely off-screen.

   Mic channel (sound pressure, always positive) keeps the default
   auto-scale anchored at zero.  Histograms (per-interval peaks, also
   always positive) likewise keep bars rising from a zero baseline.

2. Modal labels clarified to remove the 'Timestamp' vs 'Captured at'
   ambiguity:
     'Timestamp'   →  'Recorded at'         (when the seismograph
                                              recorded the event —
                                              from BW report's Event
                                              Time field)
     'Captured at' →  'Received by server at' (when our sfm-db
                                              inserted the row)
   Both have tooltips explaining the distinction.

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

sfm/sfm_webapp.html

@@ -2670,6 +2670,24 @@ function _renderScWaveform(data) {
       return String(v) + xAxisLabel;
     };
 
+    // Y-axis bounds.  Convention:
+    //   - Geophones (Tran/Vert/Long) on waveform-mode events:
+    //     symmetric around zero so the zero line sits in the middle and
+    //     positive/negative excursions are visually balanced.
+    //   - Mic (always positive sound pressure) + histograms (per-interval
+    //     peaks, always positive): default auto-scale, zero at the bottom.
+    let yBounds = {};
+    const isGeoWaveform = !isHistogram && ch !== 'MicL';
+    if (isGeoWaveform) {
+      let absMax = 0;
+      for (const v of values) {
+        const a = Math.abs(v);
+        if (a > absMax) absMax = a;
+      }
+      const padded = (absMax || 1) * 1.10;
+      yBounds = { min: -padded, max: padded };
+    }
+
     _scCharts[ch] = new Chart(canvas, {
       type: isHistogram ? 'bar' : 'line',
       data: {
@@ -2709,6 +2727,7 @@ function _renderScWaveform(data) {
             grid:  { color: '#21262d', drawTicks: showX },
           },
           y: {
+            ...yBounds,
             ticks: { color: '#484f58', maxTicksLimit: 4 },
             grid:  { color: '#21262d' },
             title: { display: true, text: chData.unit || '', color: '#484f58', font: { size: 9 } },
@@ -3072,7 +3091,8 @@ if (currentSection === 'db') {
         <h4>Event</h4>
         <dl class="sc-grid">
           <dt>Serial</dt>           <dd id="sc-f-serial">—</dd>
-          <dt>Timestamp</dt>        <dd id="sc-f-ts">—</dd>
+          <dt title="When the seismograph recorded this event (from the BW report's Event Time field)">Recorded at</dt>
+                                    <dd id="sc-f-ts">—</dd>
           <dt>Record type</dt>      <dd id="sc-f-rt">—</dd>
           <dt>Sample rate</dt>      <dd id="sc-f-sr">—</dd>
           <dt>Waveform key</dt>     <dd id="sc-f-key">—</dd>
@@ -3104,7 +3124,8 @@ if (currentSection === 'db') {
           <dt id="sc-l-bwsize">File size</dt>   <dd id="sc-f-bwsize">—</dd>
           <dt id="sc-l-sha">File sha256</dt>    <dd id="sc-f-sha">—</dd>
           <dt>Source kind</dt>      <dd id="sc-f-src">—</dd>
-          <dt>Captured at</dt>      <dd id="sc-f-cap">—</dd>
+          <dt title="When our server received and stored this event (sfm-db insert time, not the recording time)">Received by server at</dt>
+                                    <dd id="sc-f-cap">—</dd>
         </dl>
       </div>
       <div class="sc-section">