feat(email): incoming mail UI - IMAP config block + Received emails tab

Frontend for the incoming-mail feature.

- Settings -> Email: an "Incoming mail (IMAP)" block under the outgoing SMTP
  settings (same field shape: host/port/security/user/pass/folder), shown only
  when the incomingMail flag is on (IncomingMailConfigCard, self-contained
  load/save).
- A "Received emails" tab next to "Sent emails" (ReceivedEmailsPanel) listing
  the received_emails log with from/subject/received/status + attachment count
  and a link to the incoming-invoices inbox.
- `incomingMail` flag in the frontend (type + context default, standalone) +
  a Communication-section Features card.
- email.service: getIncomingConfig / updateIncomingConfig / listReceived.
- i18n: settings.features.incomingMail, email.incoming, email.received (EN+DE).

Verified: tsc --noEmit clean (0 errors); en/de JSON valid; npm run build green.

frontend/src/features/settings/tabs/FeaturesTab.tsx

@@ -6,6 +6,7 @@ import {
   Images,
   BellRing,
   MessageSquare,
+  Mailbox,
   CalendarDays,
   FileSignature,
   ScrollText,
@@ -162,6 +163,21 @@ export const FeaturesTab: React.FC = () => {
             onToggle={(next) => setFlag('reminderEmails', next)}
           />
 
+          <FeatureCard
+            icon={Mailbox}
+            title={t('settings.features.incomingMail.title', 'Incoming mail')}
+            description={t(
+              'settings.features.incomingMail.description',
+              'Poll a dedicated mailbox (IMAP) every minute and drop invoice attachments into Accounting → Incoming invoices. Configure the mailbox under Settings → Email.',
+            )}
+            status="new"
+            statusLabel={statusLabel('new')}
+            sidebarHidden
+            sidebarHiddenLabel={sidebarHiddenLabel}
+            enabled={staged.incomingMail}
+            onToggle={(next) => setFlag('incomingMail', next)}
+          />
+
           <FeatureCard
             icon={MessageSquare}
             title={t('settings.features.messaging.title', 'Messaging')}