feat: add Stalwart account security management

- Add Stalwart API client library (lib/stalwart/client.ts)
- Add server-side proxy routes for auth, crypto, password, principal, probe
- Add account security Zustand store with full state management
- Add Security settings tab with password change, display name, TOTP 2FA,
  app passwords, and encryption-at-rest controls
- Add stalwartFeaturesEnabled config flag (opt-out via STALWART_FEATURES=false)
- Add i18n translations for all 8 locales (en, de, es, fr, it, ja, nl, pt)
- Add tests for Stalwart client (24 tests) and security store (29 tests)

app/api/config/route.ts

@@ -23,6 +23,7 @@ export async function GET() {
     oauthIssuerUrl: process.env.OAUTH_ISSUER_URL || '',
     rememberMeEnabled: !!process.env.SESSION_SECRET,
     settingsSyncEnabled: process.env.SETTINGS_SYNC_ENABLED === 'true' && !!process.env.SESSION_SECRET,
+    stalwartFeaturesEnabled: process.env.STALWART_FEATURES !== 'false',
     devMode: process.env.DEV_MOCK_JMAP === 'true',
   });
 }