feat: choose how dates are displayed (Standard / French / US)

Settings → General, beside the language. An operator reading 2026-07-30 as the
7th of the 30th month is reading their own log wrongly, and that is a display
problem.

Storage stays ISO/ADIF, deliberately and permanently: it is what ADIF
specifies, it sorts correctly as text, and a stored format that followed a UI
preference would make the log unreadable the day the preference changed. Exports
and uploads are untouched.

Two details that decide whether it actually works:

  - The columns are rebuilt when the format changes. The formatters are captured
    inside AG-Grid's column definitions, so nothing else would notice and the
    table would keep the old format until something forced a rebuild.
  - main.tsx re-reads the choice after the portable prefs are pulled from the
    database. The module reads localStorage at import time, which is BEFORE that
    sync — so a copied data/ folder would have shown ISO until the next launch.

UTC is not part of the choice and never will be: a logbook is kept in UTC, and
showing local time would make the display disagree with the QSO's own record
twice a year.

frontend/src/App.tsx

@@ -100,6 +100,7 @@ import { SendSpotModal, type RecentSpotQSO } from '@/components/SendSpotModal';
 import { WinkeyerPanel, type WKStatus, type WKMacro } from '@/components/WinkeyerPanel';
 import { RotorCompass } from '@/components/RotorCompass';
 import { writeUiPref } from '@/lib/uiPref';
+import { formatDateTimeUTC } from '@/lib/dateFormat';
 import { setGridPrefsProfile, flushGridPrefs } from '@/lib/gridPrefs';
 import { DvkPanel, type DVKMsg, type DVKStat } from '@/components/DvkPanel';
 
@@ -169,11 +170,7 @@ function parseAwardRefs(s: any): Record<string, string> {
 }
 
 function fmtDateUTC(s: any): string {
-  if (!s) return '';
-  const d = new Date(s);
-  if (isNaN(d.getTime())) return s;
-  const p = (n: number) => String(n).padStart(2, '0');
-  return `${d.getUTCFullYear()}-${p(d.getUTCMonth() + 1)}-${p(d.getUTCDate())} ${p(d.getUTCHours())}:${p(d.getUTCMinutes())}`;
+  return formatDateTimeUTC(s);
 }
 function fmtFreq(hz?: number): string {
   if (!hz) return '';