fix(email): mark required fields on incoming mail to match outgoing SMTP

The IMAP card was restyled to match SMTP but didn't carry the required-field
markers. Aligned the required set (protocol differences kept):
- red asterisks on Host *, Port *, Username * (SMTP marks Host/Port/From-Email;
  IMAP has no From-Email but always needs a login)
- client-side guard mirroring handleSaveSmtp (block save without host/port/user)
- backend POST /incoming-config now requires imap_user (the poller's
  getImapConfig returns null without it)
- en/de requiredFields string

backend/src/routes/adminEmail.js

@@ -141,6 +141,9 @@ router.post('/incoming-config', [
   requirePermission('email.edit'),
   body('imap_host').notEmpty().withMessage('IMAP host is required'),
   body('imap_port').isInt({ min: 1, max: 65535 }).withMessage('Invalid port number'),
+  // IMAP always needs a login (unlike SMTP relay) — the poller's
+  // getImapConfig() returns null without a username, so require it.
+  body('imap_user').notEmpty().withMessage('IMAP username is required'),
 ], async (req, res) => {
   try {
     const errors = validationResult(req);

frontend/src/components/admin/IncomingMailConfigCard.tsx

@@ -31,7 +31,15 @@ export const IncomingMailConfigCard: React.FC = () => {
   const set = (k: keyof IncomingMailConfig, v: any) => setCfg((c) => ({ ...c, [k]: v }));
 
   const save = useMutation({
-    mutationFn: () => emailService.updateIncomingConfig(cfg),
+    mutationFn: () => {
+      // Mirror the SMTP card's client-side required guard. Host + port +
+      // username are needed for the poller to authenticate (getImapConfig
+      // returns null without host+user).
+      if (!cfg.imap_host || !cfg.imap_port || !cfg.imap_user) {
+        return Promise.reject(new Error(t('email.incoming.requiredFields', 'Host, port and username are required.')));
+      }
+      return emailService.updateIncomingConfig(cfg);
+    },
     onSuccess: () => { toast.success(t('email.incoming.savedToast', 'Incoming mail settings saved.')); qc.invalidateQueries({ queryKey: ['incoming-mail-config'] }); },
     onError: (e: any) => toast.error(e?.response?.data?.error || e?.response?.data?.errors?.[0]?.msg || e.message || 'Failed'),
   });
@@ -66,7 +74,7 @@ export const IncomingMailConfigCard: React.FC = () => {
 
       <div className="space-y-4">
         <div>
-          <label className={labelCls}>{t('email.incoming.host', 'IMAP Host')}</label>
+          <label className={labelCls}>{t('email.incoming.host', 'IMAP Host')} <span className="text-red-500">*</span></label>
           <Input
             type="text"
             value={cfg.imap_host}
@@ -78,7 +86,7 @@ export const IncomingMailConfigCard: React.FC = () => {
 
         <div className="grid grid-cols-2 gap-4">
           <div>
-            <label className={labelCls}>{t('email.incoming.port', 'Port')}</label>
+            <label className={labelCls}>{t('email.incoming.port', 'Port')} <span className="text-red-500">*</span></label>
             <Input type="number" value={cfg.imap_port} onChange={(e) => set('imap_port', parseInt(e.target.value, 10) || 0)} placeholder="993" />
           </div>
           <div>
@@ -91,7 +99,7 @@ export const IncomingMailConfigCard: React.FC = () => {
         </div>
 
         <div>
-          <label className={labelCls}>{t('email.incoming.user', 'Username')}</label>
+          <label className={labelCls}>{t('email.incoming.user', 'Username')} <span className="text-red-500">*</span></label>
           <Input
             type="text"
             value={cfg.imap_user}

frontend/src/i18n/locales/de.json

@@ -2492,6 +2492,7 @@
       "foldersDetected": "{{count}} Ordner gefunden.",
       "noFolders": "Der Server hat keine Ordner zurückgegeben.",
       "detectFailed": "Ordner konnten nicht erkannt werden.",
+      "requiredFields": "Host, Port und Benutzername sind erforderlich.",
       "savedToast": "Einstellungen für eingehende E-Mails gespeichert."
     },
     "received": {

frontend/src/i18n/locales/en.json

@@ -2065,6 +2065,7 @@
       "foldersDetected": "{{count}} folders found.",
       "noFolders": "No folders returned by the server.",
       "detectFailed": "Could not detect folders.",
+      "requiredFields": "Host, port and username are required.",
       "savedToast": "Incoming mail settings saved."
     },
     "received": {