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/CalendarSidebar.tsx

@@ -2,9 +2,10 @@ import { useMemo, useState } from "react";
 import { useLocation } from "wouter";
 import { ChevronLeft, ChevronRight, MoreVertical, Pencil, Plus, Share2, Trash2, Eye, EyeOff, Star } from "lucide-react";
 import { useCalendar } from "@/store/calendar";
-import { useSettings } from "@/store/settings";
+import { dateTimeKey, useSettings } from "@/store/settings";
 import { addMonths, isSameDay, isToday, monthGrid, startOfDay, toLocalDateOnly } from "@/lib/dates";
 import { formatMonthYear } from "@/lib/format";
+import { formatWeekday } from "@/lib/datetime";
 import { MenuItem, MenuSep, Popover, useMenu } from "@/ui/popover";
 import { confirmDialog } from "@/ui/dialog";
 import { toast } from "@/ui/toast";
@@ -16,6 +17,7 @@ export function CalendarSidebar() {
   const [location, navigate] = useLocation();
   const cal = useCalendar();
   const weekStart = useSettings((s) => s.settings.weekStart);
+  const locale = useSettings((s) => dateTimeKey(s.settings));
   const parts = location.split("/");
   const view = parts[2] || "week";
   const dateStr = parts[3];
@@ -27,10 +29,7 @@ export function CalendarSidebar() {
   const [editCal, setEditCal] = useState<Partial<Calendar> | null>(null);
   const [share, setShare] = useState<Calendar | null>(null);
   const instances = cal.instancesIn(grid[0]!, new Date(grid[41]!.getTime() + 86400000));
-  const dow = useMemo(() => {
-    const names = ["S", "M", "T", "W", "T", "F", "S"];
-    return [...Array(7)].map((_, i) => names[(weekStart + i) % 7]);
-  }, [weekStart]);
+  const dow = useMemo(() => grid.slice(0, 7).map((d) => formatWeekday(d, "narrow")), [grid, locale]);
 
   if (!cal.available) return null;
   const calendars = Object.values(cal.calendars).sort((a, b) => a.sortOrder - b.sortOrder || a.name.localeCompare(b.name));