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/styles/app.css

@@ -664,7 +664,7 @@ img { max-width: 100%; }
 .month-cell:last-child { border-right: 0; }
 .month-cell.other { background: var(--bg-sunken); color: var(--fg-faint); }
 .month-cell:hover { background: var(--bg-hover); }
-.month-cell .day-num { width: 26px; height: 26px; display: flex; align-items: center; justify-content: center; border-radius: 50%; font-size: .9em; font-weight: 500; align-self: flex-start; }
+.month-cell .day-num { min-width: 26px; height: 26px; padding: 0 4px; display: flex; align-items: center; justify-content: center; border-radius: 999px; font-size: .9em; font-weight: 500; align-self: flex-start; white-space: nowrap; }
 .month-cell.today .day-num { background: var(--accent); color: var(--accent-fg); font-weight: 700; }
 .month-cell .more { font-size: .78em; color: var(--fg-muted); padding-left: 4px; font-weight: 600; }
 .ev-chip { display: flex; align-items: center; gap: 4px; padding: 1px 6px; border-radius: 4px; font-size: .8em; line-height: 1.4; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; cursor: pointer; color: #fff; font-weight: 500; flex: 0 0 auto; border: 1px solid transparent; }