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;

e2e/electron-smoke.spec.ts

@@ -36,7 +36,7 @@ test.describe('Electron desktop shell', () => {
         // needing a reachable JMAP server just to prove the login screen
         // renders - any non-empty JMAP_SERVER_URL is enough to reach
         // "env-managed" state and serve the normal app shell.
-        JMAP_SERVER_URL: 'https://stalwart.sandbox.vnc.de',
+        JMAP_SERVER_URL: 'https://emailcore.src-advisory.com',
         SESSION_SECRET: 'electron-smoke-test-not-for-production',
         NODE_ENV: 'production',
       },