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/lib/i18n.tsx

@@ -63,7 +63,7 @@ const en: Dict = {
   // Language chooser
   'lang.choose': 'Choose your language', 'lang.chooseHint': 'You can change this later in Settings → General.',
   'lang.english': 'English', 'lang.french': 'Français', 'whatsnew.title': "What's new", 'whatsnew.close': 'Got it', 'whatsnew.none': 'No changelog available for this version yet.',
-  'settings.language': 'Language', 'settings.languageHint': 'Interface language.',
+  'settings.language': 'Language', 'gen.dateFormat': 'Date display', 'gen.dateStandard': 'Standard', 'gen.dateFR': 'French', 'gen.dateUS': 'US', 'settings.languageHint': 'Interface language.',
   'stats.tab': 'Statistics', 'stats.title': 'Logbook statistics', 'stats.loading': 'Crunching the log…',
   'stats.noData': 'No data', 'stats.charts': 'Charts', 'stats.table': 'Table', 'stats.refresh': 'Refresh',
   'stats.qsos': 'QSOs', 'stats.uniqueCalls': 'Unique callsigns', 'stats.entities': 'Entities',
@@ -475,7 +475,7 @@ const fr: Dict = {
   'lang.choose': 'Choisissez votre langue', 'lang.chooseHint': 'Modifiable plus tard dans Réglages → Général.',
   'lang.english': 'English', 'lang.french': 'Français', 'whatsnew.title': 'Nouveautés', 'whatsnew.close': 'Compris', 'whatsnew.none': 'Aucune nouveauté pour cette version pour le moment.',
   'upd.checking': 'Recherche de mises à jour…', 'upd.upToDate': 'Vous êtes à jour',
-  'settings.language': 'Langue', 'settings.languageHint': "Langue de l'interface.",
+  'settings.language': 'Langue', 'gen.dateFormat': 'Affichage des dates', 'gen.dateStandard': 'Standard', 'gen.dateFR': 'Fran\u00e7ais', 'gen.dateUS': 'US', 'settings.languageHint': "Langue de l'interface.",
   'stats.tab': 'Statistiques', 'stats.title': 'Statistiques du journal', 'stats.loading': 'Analyse du journal…',
   'stats.noData': 'Aucune donnée', 'stats.charts': 'Graphiques', 'stats.table': 'Tableau', 'stats.refresh': 'Rafraîchir',
   'stats.qsos': 'QSO', 'stats.uniqueCalls': 'Indicatifs uniques', 'stats.entities': 'Entités',