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

README.md

@@ -50,6 +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
 - 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**
@@ -64,10 +65,10 @@ browser  ──(same-origin /api/*)──►  ihasmail server (Node + Hono)  ─
   JMAP client + stores                • /api/jmap, /api/blob, /api/upload, /api/events (SSE), /api/image
 ```
 
-- `web/` — Vite + React 19 + TypeScript SPA. `src/jmap` (client, push, types), `src/store` (zustand stores: session, mail, compose, contacts, calendar, files, sieve, settings), `src/views` (mail, compose, calendar, contacts, files, settings), `src/lib` (sanitiser, search parser, Sieve codec, dates, vCard, …).
+- `web/` — Vite + React 19 + TypeScript SPA. `src/jmap` (client, push, types), `src/store` (zustand stores: session, mail, compose, contacts, calendar, files, sieve, settings), `src/views` (mail, compose, calendar, contacts, files, settings), `src/lib` (sanitiser, search parser, Sieve codec, dates and locale-aware formatting, vCard, …).
 - `server/` — tiny Node/Hono backend: authenticates against Stalwart's JMAP session endpoint, stores the credentials sealed with a key derived from the cookie secret (the server never persists plaintext passwords), proxies JMAP/blob/SSE calls, serves the SPA with a strict CSP. Also contains `src/mock/` — an in-memory fake Stalwart for local development and demos.
 
-Stalwart capabilities used: `core`, `mail`, `submission`, `vacationresponse`, `sieve`, `contacts`(+`parse`), `calendars`(+`parse`), `principals`(+`availability`), `quota`, `blob`, `filenode`, EventSource push. Features degrade gracefully when a capability is missing.
+Stalwart capabilities used: `core`, `mail`, `submission`, `vacationresponse`, `sieve`, `contacts`(+`parse`), `calendars`(+`parse`), `principals`(+`availability`), `quota`, `blob`, `filenode`, EventSource push, plus Stalwart's own `urn:stalwart:jmap` (read-only, for the account locale). Features degrade gracefully when a capability is missing.
 
 ## Quick start (Docker)
 
@@ -129,6 +130,8 @@ 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.
+- 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