fix: IMAP-Serveradresse dynamisch aus Backend laden

Hardcodierte 192.168.1.131 in settings/page.tsx ersetzt durch
dynamischen API-Call GET /api/system/info → {fqdn, imap_port}.
Fallback auf window.location.hostname wenn API nicht antwortet.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/api/system.ts

@@ -6,6 +6,12 @@ export interface HealthResponse {
   status: string;
 }
 
+export interface SystemInfo {
+  fqdn: string;
+  imap_port: number;
+  imap_port_alt: number;
+}
+
 export interface SMTPStatus {
   // global daemon fields (superadmin)
   running?: boolean;
@@ -157,6 +163,10 @@ export async function getHealth(): Promise<HealthResponse> {
   return request<HealthResponse>("/api/health");
 }
 
+export async function getSystemInfo(): Promise<SystemInfo> {
+  return request<SystemInfo>("/api/system/info");
+}
+
 export async function getSMTPStatus(): Promise<SMTPStatus> {
   return request<SMTPStatus>("/api/admin/smtp/status");
 }