Notifications that arrive when ihasmail is closed

ihasmail's notifications came from EventSource, which lives exactly as
long as a tab does -- so "desktop notifications" has always quietly
meant "while you are looking". That switch is now labelled as much, and
a second one does the thing people assumed the first one did.

Stalwart 0.16 signs Web Push with VAPID (RFC 9749) and can put the
message itself in the payload (draft-ietf-jmap-emailpush). The server
pushes straight to the browser's own push service: ihasmail's server is
not in the delivery path, there is no relay to run, and nothing beyond
the browser vendor's endpoint that Web Push requires of everyone.

Checked against the live 0.16.19 before any of this was written, because
an advertised capability is not a configured one:

  - the session publishes a real applicationServerKey, so no key
    generation or server configuration is needed
  - PushSubscription/get answers an ordinary user rather than refusing
  - emailpush is advertised, and its draft defines a filter, an ordered
    properties list and an urgency -- so the payload can carry sender and
    subject, and the server drops properties from the end when it will
    not fit rather than failing the notification

Three things this gets right that are easy to get wrong:

  - The verification handshake. A JMAP subscription delivers nothing
    until the client echoes back a code the server pushed, and the
    service worker cannot answer it -- no credentials in that context.
    It forwards the code to a tab, or leaves it in the cache when no tab
    was open to forward it to.

  - Key encoding. The W3C Push API produces unpadded base64url and
    Stalwart 0.16 was fixed to accept exactly that, so nothing here pads
    on the way out. The VAPID key needs padding on the way *in* for
    atob; getting that backwards fails at subscribe() with an opaque
    error, so it lives in one named function with tests.

  - Sign-out. A subscription belongs to the account, not the session.
    Without tearing it down, a shared machine keeps notifying for a
    mailbox nobody is signed into -- which is somebody else's mail.

The mock models the JMAP half, including refusing padded keys and
non-https endpoints, and creating subscriptions *unverified*. Delivery
cannot be mocked -- it runs through the browser vendor's real push
service -- but a mock that marked a subscription verified on creation
would let a client ship without the handshake, and the symptom in
production is "registered, and silent".

Not verified end to end: an actual notification arriving. That needs a
real browser, a real push service and real delivery, so it is live
testing or nothing.

README.md

@@ -76,6 +76,7 @@ seconds of downtime with nothing lost.
 - Search with Gmail operators (`from:`, `to:`, `subject:`, `has:attachment`, `is:unread`, `is:starred`, `in:`, `label:`, `before:`, `after:`, `larger:`, `smaller:` …) plus an advanced-search panel
 - Composer: multiple floating/minimised/maximised composers, rich-text editor (formatting, lists, links, colours, images pasted/dropped inline, emoji), plain-text mode, recipient chips with autocomplete from **contacts, the directory (GAL) and recent recipients**, multiple identities with HTML signatures, Cc/Bcc, priority, read-receipt request, templates/canned responses, attachment upload with progress, drag & drop, attachment reminder, **undo send**, **scheduled send** (quick picks or an exact date and time; the message waits in the server's queue, so it goes out whether or not ihasmail is open), autosaved drafts, reply/reply-all/forward with quoting and inline images preserved
 - Live updates via JMAP push (EventSource proxied server-side) with polling fallback; desktop notifications, sound, title/favicon unread badge
+- **Notifications that arrive with ihasmail closed** — Web Push signed with VAPID ([RFC 9749](https://datatracker.ietf.org/doc/rfc9749/)), which Stalwart 0.16 supports and without which Chromium and Safari cannot receive push at all. Stalwart pushes straight to the browser's own push service: no relay, no extra service to run, and ihasmail's server is not in the delivery path. With [`emailpush`](https://datatracker.ietf.org/doc/draft-ietf-jmap-emailpush/) the payload carries the sender and subject, so the notification is useful without a round-trip — and the filter lives on the server, so spam never leaves it. The subscription is torn down on sign-out, because it belongs to the account rather than the session and would otherwise keep notifying a shared machine for a mailbox nobody is signed into
 - A–Z folder list with Inbox pinned on top (other special folders mixed in), subfolders nested and collapsed by default with chevrons in their own gutter so every icon lines up; unread folders are bold (a parent is bold when a subfolder has unread mail); right-click a folder to mark it read *including subfolders*, create/rename/hide/share/empty, quota bar, Outlook-style module bar (Mail · Calendar · Contacts · Files) at the bottom of the pane, multi-account switching for shared accounts
 
 **Calendar** (JMAP Calendars / JSCalendar)