feat(compose): From override + catch-all auto-reply (fixes #246)

Adds an Override toggle in the composer's From row. When enabled, name
and address become free-text inputs. Mail is still submitted through the
selected identity, but the outgoing message's From: header — and the
SMTP envelope MAIL FROM when different — is set from the override.

The existing "Auto-select Reply Address" setting is extended: if the
incoming message was addressed to an alias on a domain that matches one
of your identities but isn't itself an identity (classic domain catch-
all), it now auto-enables Override and pre-fills the alias. Quick reply
honors the same resolution. The setting is relabeled to reflect the
broader behavior.

JMAP: client.sendEmail gains an optional envelopeMailFrom; when set, the
EmailSubmission includes an explicit envelope with that mailFrom and the
to/cc/bcc as rcptTo so header-From and envelope can diverge (JMAP §7.3).

S/MIME: override is incompatible with sign/encrypt and is refused with a
clear error — signing a different visible From from the identity's
certificate Subject would produce messages clients reject.

Tests: resolveReplyFrom covers exact match, sub-address stripping,
catch-all detection, identity preference, and foreign-domain null.

lib/jmap/client-interface.ts

@@ -146,6 +146,7 @@ export interface IJMAPClient {
     attachments?: Array<{ blobId: string; name: string; type: string; size: number; disposition?: 'attachment' | 'inline'; cid?: string }>,
     inReplyTo?: string[],
     references?: string[],
+    envelopeMailFrom?: string,
   ): Promise<void>;
 
   sendImipReply(opts: {