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/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",