viewers: render timestamps in browser-local time

Spotted on the SFM webapp event modal — "Received by server at" was
showing the raw ISO string "2026-05-27T21:59:57.213043Z" because we
were assigning ev.timestamp / src.captured_at directly to the
textContent of the modal fields, bypassing the existing _fmtTs()
helper that wraps them in toLocaleString().

Net effect for operators: confusing "21:59 vs it's 6 PM" mismatch
when the displayed UTC timestamp didn't match wall-clock time.  The
values were always correct; the display was just ambiguous.

After this fix:
  - "Recorded at" (naive ISO from BW = unit local time) renders
    cleanly as the unit wrote it: "5/27/2026, 6:00:13 AM"
  - "Received by server at" (UTC with Z suffix) converts to browser
    local: "5/27/2026, 5:59:57 PM"
  - Timestamp column in the history table already used _fmtTs —
    unchanged
  - Same fix applied to the standalone /events page (sidebar event
    list + meta header) via a new _fmtTsLocal helper

Note: did NOT add file-mtime-on-watcher-PC tracking as a separate
"Called in at" column — discussed and decided created_at is close
enough for schedule-compliance monitoring (worst case lag = watcher
poll interval ~60s, indistinguishable from BW write time at the
operationally-relevant resolution).

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

sfm/event_browser.html

@@ -356,6 +356,16 @@ function _psiToDbl(psi) {
   return 20 * Math.log10(psi / DBL_REF);
 }
 
+// Format an ISO timestamp in the browser's local timezone — UTC values
+// (with 'Z' suffix) convert; naive values are interpreted as local clock.
+// Returns '—' for null/empty/unparseable.
+function _fmtTsLocal(iso) {
+  if (!iso) return '—';
+  const d = new Date(iso);
+  if (isNaN(d)) return iso;
+  return d.toLocaleString();
+}
+
 // Adaptive decimal formatter — scientific notation only for truly extreme
 // values.  Normal-range peaks render as plain decimals with sensible
 // precision (was previously forcing toExponential(3) which produced ugly
@@ -458,7 +468,7 @@ function renderEventList() {
     const row = document.createElement('div');
     row.className = 'event-row' + (ev.false_trigger ? ' false_trigger' : '');
     if (ev.id === currentEventId) row.className += ' active';
-    const ts = (ev.timestamp || '').replace('T', ' ').replace('Z', '');
+    const ts = _fmtTsLocal(ev.timestamp);
     const pvs = ev.peak_vector_sum != null ? `${ev.peak_vector_sum.toFixed(3)} in/s` : '—';
     row.innerHTML = `
       <div class="er-top">
@@ -510,7 +520,7 @@ function renderMeta(data, ev) {
   const metaDiv = document.getElementById('event-meta');
   const fields = [
     ['Serial',      data.serial || ev?.serial || '—'],
-    ['Timestamp',   (data.timestamp || ev?.timestamp || '—').replace('T', ' ').replace('Z', '')],
+    ['Timestamp',   _fmtTsLocal(data.timestamp || ev?.timestamp)],
     ['Record',      data.record_type || ev?.record_type || '—'],
     ['Sample rate', data.sample_rate ? `${data.sample_rate} sps` : '—'],
     ['Geo range',   data.geo_range ? `${data.geo_range} (${data.geo_full_scale_ips} in/s FS)` : '—'],