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/contexts/FeatureFlagsContext.tsx

@@ -21,6 +21,8 @@ export const DEFAULT_FLAGS: FeatureFlags = {
   quotes: false,
   bills: false,
   messaging: false,
+  // Incoming mail (migration 128) — IMAP intake. Standalone, default off.
+  incomingMail: false,
   analytics: true,
   userManagement: true,
   // Top-level Clients section (#354 follow-up). Migration 097 mirrors