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/lib/dates.ts

@@ -1,3 +1,5 @@
+import { formatClock, formatDayMonth, formatDayMonthTime, formatWeekdayDate } from "./datetime";
+
 export const DAY_MS = 86_400_000;
 
 export function startOfDay(d: Date): Date {
@@ -219,14 +221,13 @@ export function listTimeZones(): string[] {
 export function formatTimeRange(start: Date, end: Date, allDay: boolean): string {
   if (allDay) {
     const lastDay = new Date(end.getTime() - 1);
-    if (isSameDay(start, lastDay)) return start.toLocaleDateString(undefined, { weekday: "long", month: "long", day: "numeric" });
-    return `${start.toLocaleDateString(undefined, { month: "short", day: "numeric" })} – ${lastDay.toLocaleDateString(undefined, { month: "short", day: "numeric" })}`;
+    if (isSameDay(start, lastDay)) return formatWeekdayDate(start);
+    return `${formatDayMonth(start)} – ${formatDayMonth(lastDay)}`;
   }
-  const t = (d: Date) => d.toLocaleTimeString(undefined, { hour: "numeric", minute: "2-digit" });
   if (isSameDay(start, end)) {
-    return `${start.toLocaleDateString(undefined, { weekday: "long", month: "long", day: "numeric" })} · ${t(start)} – ${t(end)}`;
+    return `${formatWeekdayDate(start)} · ${formatClock(start)} – ${formatClock(end)}`;
   }
-  return `${start.toLocaleString(undefined, { month: "short", day: "numeric", hour: "numeric", minute: "2-digit" })} – ${end.toLocaleString(undefined, { month: "short", day: "numeric", hour: "numeric", minute: "2-digit" })}`;
+  return `${formatDayMonthTime(start)} – ${formatDayMonthTime(end)}`;
 }
 
 /** For <input type="datetime-local"> */