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/dateFormat.ts

@@ -0,0 +1,91 @@
+// How dates are DISPLAYED. Nothing here touches what is stored.
+//
+// The log keeps ADIF/ISO throughout — the database, ADIF export, LoTW, every
+// upload. This is the reading layer only, because 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, not a data one.
+//
+// Storage stays ISO deliberately and permanently: it sorts correctly as text,
+// it is what ADIF specifies, and a log whose stored format followed a UI
+// preference would become unreadable the day the preference changed.
+import { writeUiPref } from '@/lib/uiPref';
+
+export type DateFormat = 'iso' | 'fr' | 'us';
+
+const KEY = 'opslog.dateFormat';
+
+function read(): DateFormat {
+	const v = localStorage.getItem(KEY);
+	return v === 'fr' || v === 'us' ? v : 'iso';
+}
+
+let current: DateFormat = read();
+
+// A minimal store so every grid and panel re-renders the moment the format
+// changes, rather than showing the old one until the next restart.
+const listeners = new Set<() => void>();
+
+export function getDateFormat(): DateFormat {
+	return current;
+}
+
+export function setDateFormat(f: DateFormat): void {
+	current = f;
+	writeUiPref(KEY, f);
+	listeners.forEach((l) => l());
+}
+
+export function subscribeDateFormat(fn: () => void): () => void {
+	listeners.add(fn);
+	return () => listeners.delete(fn);
+}
+
+// reloadDateFormat re-reads the stored value — for after the portable prefs
+// have been synced from the database at startup, which happens AFTER this
+// module's first read.
+export function reloadDateFormat(): void {
+	const v = read();
+	if (v !== current) {
+		current = v;
+		listeners.forEach((l) => l());
+	}
+}
+
+const pad = (n: number) => String(n).padStart(2, '0');
+
+// order arranges the three parts. Separators stay '-' in every format: a slash
+// in the French one would be conventional but it also invites the reader to
+// take it for a US date, which is exactly the confusion being fixed.
+function order(y: number, m: number, d: number): string {
+	switch (current) {
+		case 'fr':
+			return `${pad(d)}-${pad(m)}-${y}`;
+		case 'us':
+			return `${pad(m)}-${pad(d)}-${y}`;
+	}
+	return `${y}-${pad(m)}-${pad(d)}`;
+}
+
+// formatDateTimeUTC renders a timestamp with the time, in UTC.
+//
+// UTC is not negotiable and is not part of the preference: a logbook is kept in
+// UTC, and a date shown in local time would disagree with the QSO's own record
+// twice a year.
+export function formatDateTimeUTC(s: unknown): string {
+	if (!s) return '';
+	const d = new Date(String(s));
+	if (isNaN(d.getTime())) return String(s);
+	return `${order(d.getUTCFullYear(), d.getUTCMonth() + 1, d.getUTCDate())} ${pad(d.getUTCHours())}:${pad(d.getUTCMinutes())}`;
+}
+
+// formatDateOnly renders a date with no time. It accepts both the ADIF form
+// (YYYYMMDD, used by every QSL and upload date) and anything Date can parse.
+export function formatDateOnly(s: unknown): string {
+	if (!s) return '';
+	const t = String(s).trim();
+	const m = t.match(/^(\d{4})(\d{2})(\d{2})/);
+	if (m) return order(Number(m[1]), Number(m[2]), Number(m[3]));
+	const d = new Date(t);
+	if (isNaN(d.getTime())) return t;
+	return order(d.getUTCFullYear(), d.getUTCMonth() + 1, d.getUTCDate());
+}