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/sfm_webapp.html

@@ -2864,7 +2864,9 @@ function _renderSidecar(data) {
   };
 
   document.getElementById('sc-f-serial').textContent   = ev.serial          || '—';
-  document.getElementById('sc-f-ts').textContent       = ev.timestamp       || '—';
+  // Route through _fmtTs so the unit-local naive timestamp shows as
+  // "5/27/2026, 6:00:13 AM" instead of "2026-05-27T06:00:13".
+  document.getElementById('sc-f-ts').textContent       = _fmtTs(ev.timestamp);
   document.getElementById('sc-f-rt').textContent       = ev.record_type     || '—';
   document.getElementById('sc-f-sr').textContent       = (ev.sample_rate ?? '—') + (ev.sample_rate ? ' sps' : '');
   document.getElementById('sc-f-key').textContent      = ev.waveform_key    || '—';
@@ -2884,7 +2886,10 @@ function _renderSidecar(data) {
   document.getElementById('sc-f-bwsize').textContent   = bw.filesize != null ? `${bw.filesize} bytes` : '—';
   document.getElementById('sc-f-sha').textContent      = bw.sha256          || '—';
   document.getElementById('sc-f-src').textContent      = src.kind           || '—';
-  document.getElementById('sc-f-cap').textContent      = src.captured_at    || '—';
+  // captured_at has a "Z" suffix (UTC); _fmtTs converts to browser local
+  // — matches the BW-reported recorded-at, no more "21:59:57 vs it's 6 PM"
+  // confusion from operators reading the raw UTC value.
+  document.getElementById('sc-f-cap').textContent      = _fmtTs(src.captured_at);
 
   document.getElementById('sc-edit-ft').checked        = !!rev.false_trigger;
   document.getElementById('sc-edit-reviewer').value    = rev.reviewer || '';