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.

locales/en/common.json

@@ -531,6 +531,15 @@
       "to": "To: {recipients}"
     },
     "remove_sub_address": "Remove sub-address",
+    "from_override": {
+      "toggle_off": "Override",
+      "toggle_on": "Cancel override",
+      "toggle_tooltip": "Edit the From name and address freely. Mail is still sent through your identity — only the visible From header changes.",
+      "name_label": "From name",
+      "name_placeholder": "Name",
+      "email_label": "From email address",
+      "email_placeholder": "alias@example.com"
+    },
     "use_template": "Template",
     "save_as_template": "Save as Template",
     "validation": {
@@ -960,8 +969,8 @@
         "description": "Disable the rich text editor and send all emails as plain text only, including replies and forwards"
       },
       "auto_select_reply_identity": {
-        "label": "Auto-select Reply Address",
-        "description": "When replying, automatically switch the From address to the identity that originally received the message"
+        "label": "Reply From Received Address",
+        "description": "When replying, send from the address the message was originally sent to. Matches identities first; for domain catch-all deliveries, rewrites the From header to the alias while sending through your primary identity."
       },
       "sub_address_delimiter": {
         "label": "Sub-Address Delimiter",