Finish the README pass: stale locale method, and the two new bits of UI

The Dates & times entry still named x:Account/get as where the default locale
comes from, which this branch changed. It also never mentioned the top-bar
light/dark toggle or what About now reports about the server.

README.md

@@ -52,13 +52,15 @@ 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 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
+- **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:AccountSettings/get`, falling back to `x:Account/get`), and from the browser where the server will not say; 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
 - **Self-service credentials** in Settings › Security: change your password, manage **app passwords** (a separate password per mail app or device, revocable on its own), and turn **two-factor authentication** on or off by scanning a QR code. Enrolment codes are verified before anything is stored, so a mistyped key cannot lock you out, and switching 2FA on moves this browser's session onto a dedicated app password instead of signing you straight back out. Works against both Stalwart generations: the `x:AccountPassword` / `x:AppPassword` registry objects on 0.16+, and the `/api/account/auth` REST endpoint on 0.15.x (the latter confirmed live)
+- **Light and dark** follow the system by default, with a toggle in the top bar for flipping between them and a three-way choice in Settings › Appearance
 - 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**
 - Installable PWA (manifest + service worker), mobile layout with bottom tab bar, drawer navigation, full-screen composer, FAB
 - **Default mail app**: register ihasmail as the browser's handler for `mailto:` links from Settings › General (`registerProtocolHandler`; needs HTTPS and a browser that supports it — Safari does not). Installed as an app it also declares `protocol_handlers` in the manifest, which is what lets the operating system offer ihasmail wherever it asks for a mail client. Links arrive with recipients, Cc, Bcc, subject and body filled in
+- **About** reports the Stalwart generation ihasmail detected (0.16+ or older) and the edition where the server gives one. Stalwart does not publish a version number to clients, so no version is shown rather than a made-up one
 - Security: no credentials in the browser (server-side session with per-session encrypted upstream credentials), httpOnly SameSite cookies, CSRF header + Sec-Fetch-Site checks, strict CSP, sandboxed blob downloads, SSRF-safe image proxy, login rate limiting, security headers
 
 ## Architecture