Configurable date and time formats, defaulting to the Stalwart locale

Every user-visible date now goes through web/src/lib/datetime.ts, driven by
three settings (Settings > General > Locale):

- Language & region: automatic, or any of the 618 locales CLDR has data for,
  each named in its own language and script (web/src/lib/locales.ts, generated
  by probing Intl over the subtag space).
- Date format: automatic (locale order), 22.11.2025, 22/11/2025, 11/22/2025,
  or ISO 8601 2025-11-22.
- Time format: automatic (locale), 24-hour, or 12-hour.

Automatic takes the locale Stalwart has for the account, read best-effort at
login via x:Account/get (urn:stalwart:jmap) and passed to the client in the
session; servers without the capability, or that deny sysAccountGet to a
regular user, fall back to the browser locale. POSIX forms are normalised
(de_DE.UTF-8 -> de-DE) and script modifiers kept (sr_RS@latin -> sr-Latn-RS,
uz_UZ@cyrillic -> uz-Cyrl-UZ), while dialect/variant/currency modifiers are
dropped and a script the locale already implies is not appended.

Numerals follow the locale (22.11.2025 renders as Arabic-Indic digits under
ar-EG); ISO 8601 is the exception and pins date and clock to Latin digits so
one line never mixes digit systems.

Rewired: message list and headers, quoted reply headers, calendar (titles,
weekday and hour gutters, mini calendar, agenda, popovers, invite cards,
free/busy), contacts, files, sessions. No raw toLocale*String date calls are
left in web/src.

Native <input type="datetime-local"> pickers always follow the browser locale
and cannot be restyled by a page, so the out-of-office fields echo the entered
instant in the chosen format underneath.

Also: month-grid day labels no longer wrap when they hold a date, and the mock
server serves x:Account/get (MOCK_LOCALE, default en_US).

Closes #1

web/src/views/settings/GeneralSettings.tsx

@@ -2,6 +2,28 @@ import { useSettings } from "@/store/settings";
 import { Switch } from "@/ui/misc";
 import { browserTimeZone, listTimeZones } from "@/lib/dates";
 import { toast } from "@/ui/toast";
+import {
+  browserLocale,
+  formatClock,
+  formatDate,
+  formatFullDateTime,
+  getServerLocale,
+  localeLabel,
+  localeOptions,
+  withPrefs,
+  type DateFormat,
+} from "@/lib/datetime";
+
+/** Illustrative instant used for the format previews: 22 Nov 2025, 18:23. */
+const SAMPLE = new Date(2025, 10, 22, 18, 23);
+
+const DATE_FORMATS: Array<{ value: DateFormat; label: string }> = [
+  { value: "auto", label: "Automatic" },
+  { value: "dmy-dot", label: "Day.Month.Year" },
+  { value: "dmy-slash", label: "Day/Month/Year" },
+  { value: "mdy-slash", label: "Month/Day/Year" },
+  { value: "ymd-dash", label: "Year-Month-Day (ISO 8601)" },
+];
 
 export function GeneralSettings() {
   const s = useSettings((st) => st.settings);
@@ -9,6 +31,8 @@ export function GeneralSettings() {
   const reset = useSettings((st) => st.reset);
   const exportJson = useSettings((st) => st.exportJson);
   const importJson = useSettings((st) => st.importJson);
+  const serverLocale = getServerLocale();
+  const autoLocale = serverLocale ?? browserLocale();
 
   return (
     <div>
@@ -100,6 +124,35 @@ export function GeneralSettings() {
           </select>
         </div>
       </div>
+      <div className="field-row">
+        <div className="field">
+          <label>Language &amp; region</label>
+          <select className="select" value={s.locale} onChange={(e) => update({ locale: e.target.value })}>
+            <option value="">Automatic ({localeLabel(autoLocale)})</option>
+            {localeOptions().map((o) => <option key={o.tag} value={o.tag}>{o.label} — {o.tag}</option>)}
+          </select>
+          <p className="hint">{serverLocale ? `Your mail server reports ${localeLabel(serverLocale)} (${serverLocale}).` : "Your mail server does not report a locale, so the browser's is used."} Dates, times and month names follow this choice.</p>
+        </div>
+        <div className="field">
+          <label>Date format</label>
+          <select className="select" value={s.dateFormat} onChange={(e) => update({ dateFormat: e.target.value as DateFormat })}>
+            {DATE_FORMATS.map((f) => (
+              <option key={f.value} value={f.value}>
+                {f.label} ({withPrefs({ locale: s.locale, dateFormat: f.value }, () => formatDate(SAMPLE))})
+              </option>
+            ))}
+          </select>
+        </div>
+        <div className="field">
+          <label>Time format</label>
+          <select className="select" value={s.timeFormat} onChange={(e) => update({ timeFormat: e.target.value as typeof s.timeFormat })}>
+            <option value="auto">Automatic ({withPrefs({ locale: s.locale, timeFormat: "auto" }, () => formatClock(SAMPLE))})</option>
+            <option value="24">24-hour clock (18:23)</option>
+            <option value="12">12-hour clock (6:23 PM)</option>
+          </select>
+        </div>
+      </div>
+      <p className="hint">Preview: {formatFullDateTime(SAMPLE)}</p>
 
       <h2>Backup</h2>
       <div className="row wrap">
@@ -113,3 +166,4 @@ export function GeneralSettings() {
     </div>
   );
 }
+