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/reply-identity.ts

@@ -2,6 +2,7 @@ import type { Identity } from '@/lib/jmap/types';
 
 interface ReplyRecipient {
   email?: string | null;
+  name?: string | null;
 }
 
 interface ReplyRecipients {
@@ -29,6 +30,11 @@ function normalizeBaseEmailAddress(email: string): string {
   return `${plusIndex >= 0 ? localPart.slice(0, plusIndex) : localPart}@${domain}`;
 }
 
+function domainOf(email: string): string {
+  const at = email.indexOf('@');
+  return at > 0 ? email.slice(at + 1).toLowerCase() : '';
+}
+
 export function findReplyIdentityId(
   identities: Identity[],
   recipients?: ReplyRecipients,
@@ -59,4 +65,93 @@ export function findReplyIdentityId(
   const baseIdentity = identities.find((identity) => baseMatches.has(normalizeBaseEmailAddress(identity.email)));
 
   return baseIdentity?.id ?? null;
+}
+
+export interface ReplyFromResolution {
+  /** Identity to use for JMAP `identityId` and the SMTP envelope MAIL FROM. */
+  identityId: string;
+  /**
+   * Override for the outgoing `From:` header. Populated when the incoming
+   * message was delivered to an address on a domain the user owns (by
+   * identity) but that isn't itself a configured identity — typical
+   * domain-catch-all deployments. When set, the composer should put this
+   * address (and `overrideName`) in the message's From header while sending
+   * through the chosen identity.
+   */
+  overrideEmail?: string;
+  overrideName?: string;
+}
+
+/**
+ * Pick the identity + optional header-From override for replying to a message.
+ *
+ * Decision order:
+ *   1. If a recipient address exactly matches an identity, reply as that
+ *      identity with no override.
+ *   2. Else if a recipient matches an identity after stripping `+tag`
+ *      sub-addressing, reply as that identity with no override.
+ *   3. Else if a recipient address is on a domain that one of the identities
+ *      uses, treat that recipient as a catch-all alias: return the matching
+ *      identity + the recipient as a header-From override.
+ *   4. Else return `null` (caller falls back to primary identity).
+ */
+export function resolveReplyFrom(
+  identities: Identity[],
+  recipients?: ReplyRecipients,
+): ReplyFromResolution | null {
+  if (identities.length === 0 || !recipients) {
+    return null;
+  }
+
+  const received: { email: string; name: string | undefined }[] = [
+    ...(recipients.to || []),
+    ...(recipients.cc || []),
+    ...(recipients.bcc || []),
+  ].flatMap((r) => {
+    const email = r.email?.trim();
+    if (!email) return [];
+    return [{ email, name: r.name?.trim() || undefined }];
+  });
+
+  if (received.length === 0) {
+    return null;
+  }
+
+  const identityEmails = new Set(identities.map((i) => normalizeEmailAddress(i.email)));
+  const identityBaseEmails = new Set(identities.map((i) => normalizeBaseEmailAddress(i.email)));
+
+  const exactIdentity = identities.find((i) =>
+    received.some((r) => normalizeEmailAddress(r.email) === normalizeEmailAddress(i.email)),
+  );
+  if (exactIdentity) {
+    return { identityId: exactIdentity.id };
+  }
+
+  const baseIdentity = identities.find((i) =>
+    received.some((r) => normalizeBaseEmailAddress(r.email) === normalizeBaseEmailAddress(i.email)),
+  );
+  if (baseIdentity) {
+    return { identityId: baseIdentity.id };
+  }
+
+  const ownedDomains = new Set(identities.map((i) => domainOf(i.email)).filter(Boolean));
+
+  const catchAll = received.find((r) => {
+    const email = normalizeEmailAddress(r.email);
+    if (identityEmails.has(email) || identityBaseEmails.has(normalizeBaseEmailAddress(email))) {
+      return false;
+    }
+    return ownedDomains.has(domainOf(email));
+  });
+
+  if (catchAll) {
+    const anchor = identities.find((i) => domainOf(i.email) === domainOf(catchAll.email)) || identities[0];
+    return {
+      identityId: anchor.id,
+      overrideEmail: catchAll.email,
+      overrideName: catchAll.name,
+    };
+  }
+
+  return null;
 }