feat(email): add 'Test connection' to incoming mail + tidy IMAP label

- emailIntakeService.testConnection(): logs in, opens the configured folder,
  reports message/unread counts (non-destructive). Accepts current form creds
  so it works before saving; masked password falls back to stored.
- route POST /admin/email/incoming-config/test
- IMAP card: 'Test connection' button beside Save; toast shows folder + counts
- capitalize 'IMAP Host' label to match 'SMTP Host'

Note: incoming uses IMAP (receiving) vs outgoing SMTP (sending) — genuinely
different servers/credentials, hence the distinct field set (Folder; no From).

frontend/src/components/admin/IncomingMailConfigCard.tsx

@@ -11,7 +11,7 @@ import React, { useEffect, useState } from 'react';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { useTranslation } from 'react-i18next';
 import { toast } from 'react-toastify';
-import { Save, Server, User, Lock, Eye, EyeOff, FolderSearch } from 'lucide-react';
+import { Save, Server, User, Lock, Eye, EyeOff, FolderSearch, PlugZap } from 'lucide-react';
 import { Button, Card, Input, Loading } from '../common';
 import { emailService, type IncomingMailConfig, type ImapFolder } from '../../services/email.service';
 
@@ -44,6 +44,12 @@ export const IncomingMailConfigCard: React.FC = () => {
     onError: (e: any) => toast.error(e?.response?.data?.error || e?.response?.data?.errors?.[0]?.msg || e.message || 'Failed'),
   });
 
+  const test = useMutation({
+    mutationFn: () => emailService.testIncoming(cfg),
+    onSuccess: (r) => toast.success(t('email.incoming.testOk', 'Connected to {{folder}} — {{messages}} messages, {{unseen}} unread.', { folder: r.folder, messages: r.messages, unseen: r.unseen })),
+    onError: (e: any) => toast.error(e?.response?.data?.error || e.message || t('email.incoming.testFailed', 'Connection failed.')),
+  });
+
   const detect = useMutation({
     mutationFn: () => emailService.listIncomingFolders(cfg),
     onSuccess: (list) => {
@@ -152,9 +158,21 @@ export const IncomingMailConfigCard: React.FC = () => {
           <p className="mt-1 text-xs text-neutral-500 dark:text-neutral-400">{t('email.incoming.folderHint', 'Enter host, username and password, then Detect to list the mailbox folders.')}</p>
         </div>
 
-        <Button variant="primary" onClick={() => save.mutate()} isLoading={save.isPending} leftIcon={<Save className="w-5 h-5" />} className="w-full">
-          {t('email.incoming.save', 'Save Incoming Mail Settings')}
-        </Button>
+        <div className="flex gap-2">
+          <Button
+            variant="outline"
+            onClick={() => test.mutate()}
+            isLoading={test.isPending}
+            disabled={!cfg.imap_host || !cfg.imap_user}
+            leftIcon={<PlugZap className="w-5 h-5" />}
+            className="whitespace-nowrap"
+          >
+            {t('email.incoming.test', 'Test connection')}
+          </Button>
+          <Button variant="primary" onClick={() => save.mutate()} isLoading={save.isPending} leftIcon={<Save className="w-5 h-5" />} className="flex-1">
+            {t('email.incoming.save', 'Save Incoming Mail Settings')}
+          </Button>
+        </div>
       </div>
     </Card>
   );

frontend/src/i18n/locales/de.json

@@ -2479,6 +2479,9 @@
       "title": "Eingehende E-Mails (IMAP)",
       "subtitle": "Ein dediziertes Postfach, jede Minute abgerufen; Anhänge landen in Buchhaltung → Eingangsrechnungen.",
       "host": "IMAP-Host",
+      "test": "Verbindung testen",
+      "testOk": "Verbunden mit {{folder}} — {{messages}} Nachrichten, {{unseen}} ungelesen.",
+      "testFailed": "Verbindung fehlgeschlagen.",
       "port": "Port",
       "security": "Sicherheit",
       "ssl": "SSL/TLS",

frontend/src/i18n/locales/en.json

@@ -2051,7 +2051,10 @@
     "incoming": {
       "title": "Incoming mail (IMAP)",
       "subtitle": "A dedicated mailbox polled every minute; attachments land in Accounting → Incoming invoices.",
-      "host": "IMAP host",
+      "host": "IMAP Host",
+      "test": "Test connection",
+      "testOk": "Connected to {{folder}} — {{messages}} messages, {{unseen}} unread.",
+      "testFailed": "Connection failed.",
       "port": "Port",
       "security": "Security",
       "ssl": "SSL/TLS",

frontend/src/services/email.service.ts

@@ -95,6 +95,13 @@ export interface ImapFolder {
   specialUse: string | null;
 }
 
+export interface ImapTestResult {
+  ok: boolean;
+  folder: string;
+  messages: number;
+  unseen: number;
+}
+
 export interface ReceivedEmail {
   id: number;
   message_id: string | null;
@@ -138,6 +145,11 @@ export const emailService = {
     const response = await api.post<{ folders: ImapFolder[] }>('/admin/email/incoming-config/folders', config || {});
     return response.data.folders;
   },
+  // Test the IMAP connection: opens the configured folder, reports counts.
+  async testIncoming(config?: Partial<IncomingMailConfig>): Promise<ImapTestResult> {
+    const response = await api.post<ImapTestResult>('/admin/email/incoming-config/test', config || {});
+    return response.data;
+  },
   async listReceived(params: { page?: number; pageSize?: number } = {}): Promise<ReceivedEmailsResponse> {
     const response = await api.get<ReceivedEmailsResponse>('/admin/email/received', { params });
     return response.data;