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/i18n/locales/de.json

@@ -1603,6 +1603,10 @@
         "description": "Der Kern von PicPeak. Immer verfügbar.",
         "locked": "Galerien sind die Grundlage von PicPeak und können nicht deaktiviert werden."
       },
+      "incomingMail": {
+        "title": "Eingehende E-Mails",
+        "description": "Ruft ein dediziertes Postfach (IMAP) jede Minute ab und legt Rechnungsanhänge in Buchhaltung → Eingangsrechnungen ab. Postfach unter Einstellungen → E-Mail konfigurieren."
+      },
       "reminderEmails": {
         "title": "Erinnerungs-E-Mails",
         "description": "Automatische Vor-Event-Erinnerung an Kunden N Tage vor dem Eventdatum. Vorlagen pro Kategorie (Konzert, Firma, Hochzeit, …) unter Einstellungen → Erinnerungsvorlagen; Übersteuerung pro Event auf der Event-Detailseite."
@@ -2471,6 +2475,34 @@
       "success": "Warteschlange geleert – {{sent}} gesendet, {{failed}} fehlgeschlagen",
       "empty": "Keine ausstehenden E-Mails zum Senden"
     },
+    "incoming": {
+      "title": "Eingehende E-Mails (IMAP)",
+      "subtitle": "Ein dediziertes Postfach, jede Minute abgerufen; Anhänge landen in Buchhaltung → Eingangsrechnungen.",
+      "host": "IMAP-Host",
+      "port": "Port",
+      "security": "Sicherheit",
+      "ssl": "SSL/TLS (993)",
+      "plain": "Keine / STARTTLS (143)",
+      "user": "Benutzername",
+      "pass": "Passwort",
+      "folder": "Ordner",
+      "savedToast": "Einstellungen für eingehende E-Mails gespeichert."
+    },
+    "received": {
+      "tab": "Empfangene E-Mails",
+      "from": "Von",
+      "subject": "Betreff",
+      "received": "Empfangen",
+      "status": "Status",
+      "empty": "Noch keine empfangenen E-Mails. Eingehende E-Mails aktivieren und Postfach konfigurieren.",
+      "inbox": "Eingang",
+      "statusValue": {
+        "ingested": "Erfasst",
+        "no_attachment": "Kein Anhang",
+        "duplicate": "Duplikat",
+        "error": "Fehler"
+      }
+    },
     "sentEmails": {
       "tab": "Gesendete E-Mails",
       "title": "Gesendete E-Mails",

frontend/src/i18n/locales/en.json

@@ -1161,6 +1161,10 @@
         "description": "The core PicPeak surface. Always available.",
         "locked": "Galleries are the foundation of PicPeak and can't be turned off."
       },
+      "incomingMail": {
+        "title": "Incoming mail",
+        "description": "Poll a dedicated mailbox (IMAP) every minute and drop invoice attachments into Accounting → Incoming invoices. Configure the mailbox under Settings → Email."
+      },
       "reminderEmails": {
         "title": "Reminder Emails",
         "description": "Automatic pre-event nudge to customers N days before their event date. Per-category templates (concert, corporate, wedding, …) editable in Settings → Reminder templates; per-event override on the event detail page."
@@ -2044,6 +2048,34 @@
       "success": "Email queue flushed — {{sent}} sent, {{failed}} failed",
       "empty": "No pending emails to send"
     },
+    "incoming": {
+      "title": "Incoming mail (IMAP)",
+      "subtitle": "A dedicated mailbox polled every minute; attachments land in Accounting → Incoming invoices.",
+      "host": "IMAP host",
+      "port": "Port",
+      "security": "Security",
+      "ssl": "SSL/TLS (993)",
+      "plain": "None / STARTTLS (143)",
+      "user": "Username",
+      "pass": "Password",
+      "folder": "Folder",
+      "savedToast": "Incoming mail settings saved."
+    },
+    "received": {
+      "tab": "Received emails",
+      "from": "From",
+      "subject": "Subject",
+      "received": "Received",
+      "status": "Status",
+      "empty": "No received emails yet. Enable incoming mail and configure the mailbox.",
+      "inbox": "inbox",
+      "statusValue": {
+        "ingested": "Ingested",
+        "no_attachment": "No attachment",
+        "duplicate": "Duplicate",
+        "error": "Error"
+      }
+    },
     "sentEmails": {
       "tab": "Sent emails",
       "title": "Sent emails",