config: point to new Stalwart backend (emailcore.src-advisory.com)

- JMAP_SERVER_URL: stalwart.sandbox.vnc.de → emailcore.src-advisory.com
- Updated Electron defaults, deploy secrets example, and e2e tests
- SMTP server (emailcore-svc.src-advisory.com) is handled by Stalwart
  internally via JMAP EmailSubmission — no frontend changes needed

electron/main.ts

@@ -96,7 +96,7 @@ function getServerDataDirs(): Record<string, string> {
  */
 function getDesktopDefaults(): Record<string, string> {
   return {
-    JMAP_SERVER_URL: "https://stalwart.sandbox.vnc.de",
+    JMAP_SERVER_URL: "https://emailcore.src-advisory.com",
     APP_NAME: "VNCmail+",
     APP_SHORT_NAME: "VNCmail+",
     LOGIN_LOGO_LIGHT_URL: "/branding/SRC_Symbol.png",