Custom date and time pickers that follow the configured format

Browsers render <input type="date"> and datetime-local in their own locale and
ignore the page's, so #1 left a German user on an English browser reading
22.11.2025 everywhere but still entering dates through an mm/dd/yyyy widget.
#3 makes the case that people use the picker rather than typing, which is
where the AM/PM mistakes happen.

New DateField and DateTimeField (web/src/ui/datefield.tsx) replace all nine
native controls — event editor (all-day and timed start/end, recurrence
until), out-of-office, contact birthday, advanced search. They take and emit
the same ISO strings the native inputs did, so call sites barely changed.

Each is a text box in the configured order plus a popover: a month grid
(week start from settings, locale weekday and month names, today and the
selection marked) and, for date-times, a list of times in the configured
clock. Keyboard: arrows move by day, PageUp/PageDown by month, Home/End
across the week, Enter picks, Escape closes, ArrowDown opens; the focused day
holds DOM focus so screen readers follow, and the dialog has an accessible
name (Popover gained an ariaLabel prop).

Text entry is lenient — the configured order with any separator, unseparated
digits (221125), day and month alone, non-Latin digits, and bare ISO always;
times take 18:23, 1823, 6:23pm, 930. What will not parse reverts on blur
rather than clearing the field, and impossible dates like 31 February are
rejected instead of rolling into March.

Editable boxes stay Gregorian and Latin-digit even where display does not
(fa-IR, th-TH, ar-EG): the locale's field order and separator are kept, but a
Buddhist-era year in a text box cannot round-trip against a Gregorian grid.
Noted in the README.

The out-of-office format echo added in #2 is gone — the fields now show the
right format themselves.

Closes #3

README.md

@@ -50,7 +50,7 @@ ihasmail is a JMAP-first web client: mail, calendars, contacts, files, filters a
 - Browse folders, upload (drag & drop), download, create folders, rename, move, delete
 
 **Settings**
-- **Dates & times**: language/region (every one of the ~620 locales CLDR has data for, each named in its own language and script), date order (locale default, `22.11.2025`, `22/11/2025`, `11/22/2025` or ISO `2025-11-22`) and 12h/24h clock, applied everywhere — message list and headers, calendar, contacts, files, sessions. The default comes from the locale configured for the account in Stalwart (`x:Account/get`), falling back to the browser's; POSIX forms are normalised (`de_DE.UTF-8` → `de-DE`) and script modifiers preserved (`sr_RS@latin` → `sr-Latn-RS`). Numerals follow the locale (`٢٢.١١.٢٠٢٥` for `ar-EG`), except under ISO 8601, which pins date *and* clock to Latin digits
+- **Dates & times**: language/region (every one of the ~620 locales CLDR has data for, each named in its own language and script), date order (locale default, `22.11.2025`, `22/11/2025`, `11/22/2025` or ISO `2025-11-22`) and 12h/24h clock, applied everywhere — message list and headers, calendar, contacts, files, sessions. The default comes from the locale configured for the account in Stalwart (`x:Account/get`), falling back to the browser's; POSIX forms are normalised (`de_DE.UTF-8` → `de-DE`) and script modifiers preserved (`sr_RS@latin` → `sr-Latn-RS`). Numerals follow the locale (`٢٢.١١.٢٠٢٥` for `ar-EG`), except under ISO 8601, which pins date *and* clock to Latin digits. Dates are **entered** through custom pickers in the same format (browsers render `<input type="date">` in their own locale and ignore the page's), with a calendar popover, a time list, keyboard navigation, and lenient typing — `22.11.`, `221125`, `6:23pm` and bare ISO all parse
 - Identities & signatures, **Sieve filters** (visual rule builder that round-trips to a Sieve script, plus a raw script editor with server-side validation), out-of-office (`VacationResponse`), folders, labels, templates, notifications, calendar defaults, sessions (sign out other devices), keyboard shortcuts, import/export of settings
 
 **Platform**
@@ -130,7 +130,7 @@ Verified against the mock server and, for the core mail flows, against a live St
 - **HTML signatures** — Stalwart caps identity signatures at 2 KB. ihasmail compacts pasted HTML, moves images to Files and, if still too large, keeps the full signature in Files behind a short marker (other clients see a text fallback). The end-to-end flow (save → compose → send with inline logo) is implemented but not yet confirmed on the live server.
 - **Files** — the live server runs an older Stalwart build than `main`; `FileNode/query` there rejects `isTopLevel`/`parentId` filters, so ihasmail falls back to listing all nodes and building the tree client-side. Upload/rename/move/delete still need a live pass.
 - Recurring events: colour/category/edit/delete apply to the whole series (per-occurrence overrides aren't supported by the server yet).
-- Date **pickers** (`<input type="datetime-local">` in the event editor and out-of-office settings) are native browser controls and always follow the browser's own locale — no page can restyle them. The chosen format is echoed underneath the out-of-office fields so the entered instant is unambiguous.
+- Editable date boxes are always Gregorian and in Latin digits, even for locales whose *display* uses another calendar or numbering system (`fa-IR`, `th-TH`, `ar-EG`) — they keep the locale's field order and separator, but a Buddhist-era year in a text box does not round-trip against the Gregorian calendar grid. Non-Gregorian calendar support is not implemented.
 - The account locale is read with Stalwart's `x:Account/get`, which needs the `sysAccountGet` permission; where a regular user is not granted it, ihasmail silently falls back to the browser locale and the setting can be chosen by hand.
 
 ## Roadmap / not yet