fix(admin): SMTP port can be cleared and saves without 'Ungültiges JSON'

The UI sent the port as a number while the server expects text, so every
save failed; the field also jumped back to 587 when emptied. The port is
now edited as text (digits only), and the server accepts number or text.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/settings/AdminSettingsView.tsx

@@ -96,7 +96,7 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
     allowRegistration: false,
     instanceName: 'MindSift',
     smtpHost: '',
-    smtpPort: 587,
+    smtpPort: '587',
     smtpUser: '',
     smtpPass: '',
     smtpFrom: '',
@@ -126,7 +126,7 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
           allowRegistration: Boolean(sys.allowRegistration),
           instanceName: sys.instanceName || 'MindSift',
           smtpHost: sys.smtpHost || '',
-          smtpPort: sys.smtpPort || 587,
+          smtpPort: sys.smtpPort ? String(sys.smtpPort) : '587',
           smtpUser: sys.smtpUser || '',
           smtpPass: sys.smtpPass || '',
           smtpFrom: sys.smtpFrom || '',
@@ -1150,10 +1150,12 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
                     SMTP Port (`SMTP_PORT`)
                   </label>
                   <input
-                    type="number"
-                    value={settings.smtpPort || 587}
+                    type="text"
+                    inputMode="numeric"
+                    value={settings.smtpPort ?? ''}
                     onChange={(e) =>
-                      setSettings((prev) => ({ ...prev, smtpPort: Number(e.target.value) }))
+                      // Digits only; an empty field stays empty while typing (saved as 587)
+                      setSettings((prev) => ({ ...prev, smtpPort: e.target.value.replace(/\D/g, '').slice(0, 5) }))
                     }
                     placeholder="587"
                     className="w-full h-9 px-3 input-field font-mono"

frontend/src/lib/types.ts

@@ -161,7 +161,7 @@ export interface SystemSettings {
   allowRegistration: boolean;
   instanceName: string;
   smtpHost?: string;
-  smtpPort?: number;
+  smtpPort?: string; // sent as text, like the server stores it
   smtpUser?: string;
   smtpPass?: string;
   smtpFrom?: string;