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

e2e/electron-live-sandbox.spec.ts

@@ -16,7 +16,7 @@ import path from 'node:path';
 //      real JMAP server round trip works end-to-end, without ever using or
 //      guessing a real account's credentials.
 const projectRoot = path.resolve(__dirname, '..');
-const SANDBOX_URL = 'https://stalwart.sandbox.vnc.de';
+const SANDBOX_URL = 'https://emailcore.src-advisory.com';
 
 test.describe('Electron desktop shell - live sandbox connectivity', () => {
   let electronApp: ElectronApplication;