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/components/SettingsModal.tsx

@@ -69,6 +69,7 @@ import {
 } from '@/components/ui/select';
 import { cn } from '@/lib/utils';
 import { writeUiPref } from '@/lib/uiPref';
+import { getDateFormat, setDateFormat, type DateFormat } from '@/lib/dateFormat';
 import { useI18n, FlagGB, FlagFR, type Lang } from '@/lib/i18n';
 import { useTheme, CONCRETE_THEMES, type ThemeChoice } from '@/lib/theme';
 import { OperatingPanel } from '@/components/OperatingPanel';
@@ -1143,6 +1144,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
   // a trace already running came back unticked; the operator ticks the box to
   // turn it on, which turns it off, and the log they send has no trace in it.
   // Reported by a Xiegu user. Hydrated from the backend below.
+  const [dateFmtSel, setDateFmtSel] = useState<DateFormat>(getDateFormat);
   const [wkTrace, setWkTrace] = useState(false);
   const [civTrace, setCivTrace] = useState(false);
   useEffect(() => {
@@ -5222,6 +5224,27 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
             </div>
           </div>
 
+          {/* Display only. The log is stored in ISO/ADIF whatever is chosen here
+              — that is what ADIF specifies and what sorts correctly, and a
+              stored format that followed a preference would make the log
+              unreadable the day the preference changed. */}
+          <div className="flex items-center gap-3">
+            <Label className="text-sm w-40">{t('gen.dateFormat')}</Label>
+            <div className="inline-flex rounded-md border border-border overflow-hidden">
+              {([['iso', t('gen.dateStandard'), '2026-07-30 14:25'],
+                 ['fr', t('gen.dateFR'), '30-07-2026 14:25'],
+                 ['us', t('gen.dateUS'), '07-30-2026 14:25']] as [DateFormat, string, string][]).map(([code, label, sample]) => (
+                <button key={code} type="button" title={sample}
+                  onClick={() => { setDateFormat(code); setDateFmtSel(code); }}
+                  className={cn('flex flex-col items-start px-3 py-1 text-sm font-medium border-l border-border first:border-l-0',
+                    dateFmtSel === code ? 'bg-primary text-primary-foreground' : 'bg-card text-muted-foreground hover:bg-muted')}>
+                  {label}
+                  <span className="text-[10px] font-mono opacity-70">{sample}</span>
+                </button>
+              ))}
+            </div>
+          </div>
+
           <ThemeSelector />
 
           <label className="flex items-center gap-2 text-sm cursor-pointer">