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

server/src/app.ts

@@ -11,6 +11,7 @@ import {
   expandTemplate,
   fetchUpstreamSession,
   forgetUpstreamSession,
+  getAccountLocale,
   getUpstreamSession,
   localizeSession,
 } from "./upstream.js";
@@ -170,7 +171,8 @@ export function createApp(): Hono<Env> {
         ip,
       });
       setSessionCookie(c, cookie, session.remember);
-      return c.json(localizeSession(upstream, sessionExtras(session)));
+      const locale = await getAccountLocale(session.id, session.authorization, upstream);
+      return c.json(localizeSession(upstream, sessionExtras(session, locale)));
     } catch (err) {
       return upstreamFailure(c, err);
     }
@@ -180,7 +182,8 @@ export function createApp(): Hono<Env> {
     const session = c.get("session");
     try {
       const upstream = await getUpstreamSession(session.id, session.authorization, c.req.query("refresh") === "1");
-      return c.json(localizeSession(upstream, sessionExtras(session)));
+      const locale = await getAccountLocale(session.id, session.authorization, upstream);
+      return c.json(localizeSession(upstream, sessionExtras(session, locale)));
     } catch (err) {
       if (err instanceof UpstreamError && err.status === 401) {
         sessions.destroy(session.id);
@@ -350,7 +353,7 @@ export function createApp(): Hono<Env> {
   return app;
 }
 
-function sessionExtras(session: LiveSession) {
+function sessionExtras(session: LiveSession, userLocale: string | null = null) {
   return {
     ihasmail: {
       appName: config.appName,
@@ -359,6 +362,8 @@ function sessionExtras(session: LiveSession) {
       sessionId: session.id,
       loginName: session.username,
       remember: session.remember,
+      /** Locale configured for the account in Stalwart's directory, if readable. */
+      userLocale,
     },
   };
 }