Hold a message in the server's queue until the time you asked for

Scheduled send, which the README listed as needing server support that
Stalwart has had all along. The delay cannot be asked for directly --
RFC 8621 makes `sendAt` read-only and server-derived -- so it goes on the
envelope as an RFC 4865 `HOLDUNTIL` parameter, and the server reports back
the time it settled on.

Stalwart advertises this in the *account* capability, not the session-level
one (which is empty): `maxDelayedSend` of thirty days and `FUTURERELEASE`
among its `submissionExtensions`. The composer offers scheduling only when
both are there, and never offers a time the server would refuse.

A held message goes to a Scheduled folder rather than Sent, because
`onSuccessUpdateEmail` would otherwise file it as sent the moment the
submission is created, and it has not been sent. Nothing moves it out when
the hold expires, so the folder is reconciled on the way in: released
messages to Sent, cancelled ones back to Drafts. Cancelling uses a separate
`Email/set` rather than `onSuccessUpdateEmail`, whose key Stalwart reads as
an Email id and not, as the RFC says, a submission id.

The mock grows the whole lifecycle, and learns to resolve creation
references while it is there -- it had been quietly declining to create any
submission at all, since sending names its message as `#m`. Because
Stalwart's own `futureRelease` setting defaults to off and then drops the
hold in silence, `npm run dev:mock:no-future-release` reproduces that.

Verified end to end against the mock; not yet against the live server.

web/src/jmap/client.ts

@@ -123,6 +123,16 @@ export class JmapClient {
     return Boolean(acc && cap in acc.accountCapabilities);
   }
 
+  /**
+   * The capability object itself, for the capabilities that carry limits.
+   * Stalwart puts the interesting half of `urn:ietf:params:jmap:submission`
+   * here and leaves the session-level copy empty.
+   */
+  accountCapability<T>(accountId: Id, cap: string): T | undefined {
+    const acc = this.session?.accounts[accountId];
+    return acc?.accountCapabilities[cap] as T | undefined;
+  }
+
   primaryAccount(cap: string): Id | null {
     return this.session?.primaryAccounts[cap] ?? null;
   }