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/calendar/CalendarContextMenu.tsx

@@ -3,6 +3,7 @@ import { useLocation } from "wouter";
 import type { CalendarEvent } from "@/jmap/types";
 import { useCalendar, type EventInstance } from "@/store/calendar";
 import { useSettings } from "@/store/settings";
+import { formatDayMonth } from "@/lib/datetime";
 import { MenuItem, MenuSep, MenuTitle, Popover, type Anchor } from "@/ui/popover";
 import { CALENDAR_COLORS } from "@/ui/misc";
 import { confirmDialog } from "@/ui/dialog";
@@ -48,7 +49,7 @@ export function CalendarContextMenu({ ctx, onClose, onOpen, onEdit, onCreate }:
     const { start, end, allDay } = ctx;
     return (
       <Popover anchor={ctx.anchor} onClose={onClose} width={240}>
-        <MenuItem icon={<Plus size={16} />} label={allDay ? `New all-day event on ${start.toLocaleDateString(undefined, { month: "short", day: "numeric" })}` : `New event at ${formatTime(start)}`} onClick={() => onCreate(start, end, allDay)} />
+        <MenuItem icon={<Plus size={16} />} label={allDay ? `New all-day event on ${formatDayMonth(start)}` : `New event at ${formatTime(start)}`} onClick={() => onCreate(start, end, allDay)} />
         {!allDay && <MenuItem icon={<CalendarDays size={16} />} label="New all-day event" onClick={() => { const d = new Date(start); d.setHours(0, 0, 0, 0); onCreate(d, new Date(d.getTime() + 86400000), true); }} />}
         <MenuSep />
         <MenuItem icon={<CalIcon size={16} />} label="Go to day" onClick={() => navigate(`/calendar/day/${toLocalDateOnly(start)}`)} />