feat(compose): preselect identity of the active mailbox for new messages

Starting a new message while viewing a specific mailbox/account now defaults
the From identity to that mailbox instead of the global primary identity, so
composing from info@ sends as info@. Mirrors the existing reply-time identity
match and rides the same autoSelectReplyIdentity setting; reply/replyAll/forward
keep resolving from the original recipients. Matches exact then +tag-stripped.

Extracts findComposeIdentityId into lib/reply-identity.ts with unit tests.

app/(main)/[locale]/page.tsx

@@ -3084,6 +3084,11 @@ export default function Home() {
                 <EmailComposer
                   key={composerSessionId}
                   mode={pendingDraft?.mode ?? composerMode}
+                  composeFromAccountEmail={
+                    useAccountStore
+                      .getState()
+                      .getAccountById(viewingAccountId ?? activeAccountId ?? '')?.email
+                  }
                   replyTo={pendingDraft !== null ? pendingDraft.replyTo : (selectedEmail ? {
                     from: selectedEmail.from,
                     replyToAddresses: selectedEmail.replyTo,