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/WorkedBeforeGrid.tsx

@@ -14,6 +14,7 @@ import { Checkbox } from '@/components/ui/checkbox';
 import { Badge } from '@/components/ui/badge';
 import type { WorkedBeforeView, QSOForm } from '@/types';
 import { makeColCatalog, GROUP_ORDER, groupLabel } from './RecentQSOsGrid';
+import { getDateFormat, subscribeDateFormat } from '@/lib/dateFormat';
 import { QSOContextMenu, type QSOMenuState } from './QSOContextMenu';
 import { loadLocal, loadRemote, saveState, seedLocal, whenGridPrefsReady } from '@/lib/gridPrefs';
 import { useI18n } from '@/lib/i18n';
@@ -63,7 +64,9 @@ export function WorkedBeforeGrid({ wb, myGrid, busy, currentCall, onRowDoubleCli
   const [menu, setMenu] = useState<QSOMenuState>(null);
 
   // Localized column catalog (shared with the Recent QSOs grid).
-  const COL_CATALOG = useMemo(() => makeColCatalog(t, myGrid), [t, myGrid]);
+  const [dateFmt, setDateFmt] = useState(getDateFormat);
+  useEffect(() => subscribeDateFormat(() => setDateFmt(getDateFormat())), []);
+  const COL_CATALOG = useMemo(() => makeColCatalog(t, myGrid), [t, myGrid, dateFmt]);
 
   function handleRowDoubleClicked(e: RowDoubleClickedEvent<WorkedEntry>) {
     if (e.data && onRowDoubleClicked) onRowDoubleClicked(e.data);