Fix admin interface issues and 404 errors

- Create adminEvents.js router to handle /api/admin/events endpoints
- Mount events router in admin.js to fix 404 errors
- Fix admin layout CSS - changed from static to flex layout
- Update AdminSidebar positioning from static to relative
- Add missing PUT endpoints for general and security settings
- Fix frontend environment variables in docker-compose.local.yml
- Add build args to Dockerfile.dev for environment variables
- Update CORS to accept requests from all dev servers
- Remove unused imports from SettingsPage

This fixes:
- Events page 404 error
- Admin layout misalignment (sidebar and content on different rows)
- Settings page not loading
- CORS issues between frontend and backend

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/routes/admin.js

@@ -6,11 +6,13 @@ const dashboardRoutes = require('./adminDashboard');
 const archiveRoutes = require('./adminArchives');
 const emailRoutes = require('./adminEmail');
 const settingsRoutes = require('./adminSettings');
+const eventsRoutes = require('./adminEvents');
 
 // Mount sub-routers
 router.use('/dashboard', dashboardRoutes);
 router.use('/archives', archiveRoutes);
 router.use('/email', emailRoutes);
 router.use('/settings', settingsRoutes);
+router.use('/events', eventsRoutes);
 
 module.exports = router;