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/mail/MessageList.tsx

@@ -3,7 +3,7 @@ import { useVirtualizer } from "@tanstack/react-virtual";
 import { Archive, ArrowLeft, CheckSquare, FolderInput, PanelRight, PanelBottom, PanelTop, Filter, Inbox, Mail, MailOpen, MoreVertical, Paperclip, RefreshCw, Reply, Search, Star, Tag, Trash2, AlertOctagon, Forward, Eraser, ShieldCheck } from "lucide-react";
 import { useLocation } from "wouter";
 import { useMail, type ListState } from "@/store/mail";
-import { useSettings } from "@/store/settings";
+import { dateTimeKey, useSettings } from "@/store/settings";
 import type { Email, Id } from "@/jmap/types";
 import { formatListDate } from "@/lib/format";
 import { displayName, shortName } from "@/lib/address";
@@ -327,6 +327,8 @@ interface RowProps {
 
 const Row = memo(function Row({ email: e, threadEmails, top, height, selected, focused, open, twoLine, showAvatar, showPreview, isDrafts, isSent, mailboxId, selectedIds, onClick, onContext, onSelect, onStar, onArchive, onTrash, onRead }: RowProps) {
   const labels = useSettings((s) => s.settings.labels);
+  // Subscribed purely so the row re-renders when the date format changes.
+  useSettings((s) => dateTimeKey(s.settings));
   const inScope = threadEmails ? threadEmails.filter((x) => (mailboxId ? x.mailboxIds[mailboxId] : true)) : [e];
   const scope = inScope.length ? inScope : [e];
   const unread = scope.some((x) => !x.keywords.$seen);