feat(260729-d3k): multi-line Base-DN textarea + reworked scope i18n

- Base-DN admin field is now a multi-line textarea (one DN per line),
  value stays a single newline-separated string, no schema change
- baseDnHint key added (de/en) explaining the Base-DN(s) sync scope
- groupFilter.description/emptyMeansAll reworded: group filter is an
  optional extra restriction; empty selection means all users under
  the base DN(s) are synced (drops the old "nothing is synced"
  framing)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/app/(portal)/admin/ldap/page.tsx

@@ -463,14 +463,15 @@ export default function AdminLdapPage() {
               <label className="text-sm font-medium text-foreground">
                 {t('baseDn')}
               </label>
-              <input
-                type="text"
+              <textarea
                 value={formData.baseDn}
                 onChange={(e) => setFormData({ ...formData, baseDn: e.target.value })}
-                placeholder="dc=example,dc=com"
-                className="flex h-10 w-full rounded-md border border-input bg-background px-3 py-2 text-sm"
+                placeholder={'dc=example,dc=com\nou=extern,dc=example,dc=com'}
+                rows={3}
+                className="flex min-h-20 w-full rounded-md border border-input bg-background px-3 py-2 text-sm"
                 required
               />
+              <p className="text-xs text-muted-foreground">{t('baseDnHint')}</p>
             </div>
             <div className="space-y-2">
               <label className="text-sm font-medium text-foreground">