feat: implement OAuth 2.0 / OpenID Connect support

- Added OAuth configuration management in the admin panel.
- Implemented OAuth authorization flow with PKCE for enhanced security.
- Created routes for handling OAuth provider discovery, authorization, and callback.
- Integrated OAuth login and registration options in the frontend.
- Updated UI components to support OAuth login and registration.
- Added internationalization strings for OAuth-related messages.
- Implemented encryption for client secrets and secure state management.
- Added error handling and user feedback for OAuth processes.

server/index.js

@@ -16,6 +16,7 @@ import federationRoutes, { wellKnownHandler } from './routes/federation.js';
 import calendarRoutes from './routes/calendar.js';
 import caldavRoutes from './routes/caldav.js';
 import notificationRoutes from './routes/notifications.js';
+import oauthRoutes from './routes/oauth.js';
 import { startFederationSync } from './jobs/federationSync.js';
 
 const __filename = fileURLToPath(import.meta.url);
@@ -71,6 +72,7 @@ async function start() {
   app.use('/api/federation', federationRoutes);
   app.use('/api/calendar', calendarRoutes);
   app.use('/api/notifications', notificationRoutes);
+  app.use('/api/oauth', oauthRoutes);
   // CalDAV — mounted outside /api so calendar clients use a clean path
   app.use('/caldav', caldavRoutes);
   // Mount calendar federation receive also under /api/federation for remote instances