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/contacts/ContactsView.tsx

@@ -5,6 +5,7 @@ import { useContacts } from "@/store/contacts";
 import { useCompose } from "@/store/compose";
 import type { AddressBook, ContactCard } from "@/jmap/types";
 import { contactDisplayName, contactEmails, contactPhoto, formatAddressLines, sortKey, toVCard } from "@/lib/contacts";
+import { formatDate, formatDateLong } from "@/lib/datetime";
 import { Avatar, Empty, Spinner, useIsNarrow } from "@/ui/misc";
 import { MenuItem, MenuSep, Popover, useMenu } from "@/ui/popover";
 import { confirmDialog, promptDialog } from "@/ui/dialog";
@@ -237,14 +238,14 @@ function ContactDetail({ card: c, onBack, onEdit, narrow, onEmail }: { card: Con
         </div>
       )}
       {c.keywords && Object.keys(c.keywords).length > 0 && <div className="row wrap gap-4 mt-8">{Object.keys(c.keywords).map((k) => <span key={k} className="chip"><Pin size={12} /> {k}</span>)}</div>}
-      {c.updated && <p className="hint mt-16"><CalIcon size={12} /> Updated {new Date(c.updated).toLocaleDateString()}</p>}
+      {c.updated && <p className="hint mt-16"><CalIcon size={12} /> Updated {formatDate(new Date(c.updated))}</p>}
     </div>
   );
 }
 
 function fmtPartial(d: { year?: number; month?: number; day?: number; utc?: string }): string {
-  if (d.utc) return new Date(d.utc).toLocaleDateString();
-  if (d.year && d.month && d.day) return new Date(d.year, d.month - 1, d.day).toLocaleDateString(undefined, { year: "numeric", month: "long", day: "numeric" });
-  if (d.month && d.day) return new Date(2000, d.month - 1, d.day).toLocaleDateString(undefined, { month: "long", day: "numeric" });
+  if (d.utc) return formatDate(new Date(d.utc));
+  if (d.year && d.month && d.day) return formatDateLong(new Date(d.year, d.month - 1, d.day));
+  if (d.month && d.day) return formatDateLong(new Date(2000, d.month - 1, d.day), false);
   return [d.year, d.month, d.day].filter(Boolean).join("-");
 }