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/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;