Hide identities from the compose picker

An account using a unique address per service, on a server with an alias
domain, ends up with every local part twice over and a From picker
nobody can use -- while only ever sending from a handful (#73).

Identities can now be hidden from that picker, from Identities &
signatures. Hiding is presentation only: the identity still exists,
still receives, and stays listed and editable, the way an unsubscribed
folder is still a folder. That framing is mbunkus's own, and it is the
right one -- this is a UI preference, not a change to the account.

Three things it refuses to do, because a sender picker with nothing
usable in it is worse than a cluttered one:

  - it will not hide the identity a draft is already using, which would
    leave the select with no matching option and move the From line
    under the writer
  - it will not hide the default, which is what a new draft starts on;
    the button is disabled there and says why
  - if every identity is somehow hidden -- reachable only through
    settings sync, since the UI will not do it -- they are all offered
    again

The setting syncs, so the picker looks the same on every device, which
follows from DEVICE_KEYS being a list of exceptions rather than a list
of what travels.

Verified against the mock with four identities and one hidden: the
picker offers the other three, the hidden address is gone from
composing, the default's hide button is disabled, and the row says the
identity still receives.

README.md

@@ -96,6 +96,7 @@ seconds of downtime with nothing lost.
 - **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. Built on the `x:AccountPassword` / `x:AppPassword` registry objects
 - **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
+- **Hide identities from the compose picker** — an account with alias domains can have every address twice over while only a handful are ever sent from, which makes the From picker unusable. Hiding is presentation only: the identity still exists, still receives, and stays listed and editable in Settings, the way an unsubscribed folder is still a folder. The identity a draft is already using and the default can never be hidden, and hiding every one of them offers them all again — a sender picker with nothing in it is worse than a cluttered one
 - 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
 - **Settings follow the account, not the browser**: they are kept in a `settings.json` in the account's own JMAP Files, so the default identity, locale, date and time formats, theme, labels, templates, folder colours and the rest are the same wherever you sign in — including a private window. ihasmail still stores nothing itself; the file lives in the mail store and is backed up with it. Settings that describe *this* screen or browser stay local, because syncing them would be wrong rather than helpful: list-pane sizes, density, font size, sidebar state, and the notification toggles (which track a permission the browser grants per-device). localStorage is kept as a cache so the first frame is already right, and the file corrects it a moment later