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/event_browser.html

@@ -629,6 +629,22 @@ function renderWaveform(data) {
       return (Number.isInteger(v) ? String(v) : v.toFixed(1)) + xAxisUnit;
     };
 
+    // Y-axis bounds.  Geophone waveforms render symmetric around zero
+    // (seismograph convention — zero line in the middle, signal goes
+    // up AND down).  Mic + histograms keep default auto-scale (always
+    // positive values; 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 };
+    }
+
     const chart = new Chart(canvas, {
       type: isHistogram ? 'bar' : 'line',
       data: {
@@ -677,6 +693,7 @@ function renderWaveform(data) {
             grid: { color: isPrintMode ? '#e0e0e0' : '#21262d', drawTicks: showXAxis },
           },
           y: {
+            ...yBounds,
             ticks: { color: isPrintMode ? '#666' : '#484f58', maxTicksLimit: 5 },
             grid: { color: isPrintMode ? '#e0e0e0' : '#21262d' },
             title: { display: true, text: unit,