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>

frontend/Dockerfile.dev

@@ -3,6 +3,16 @@ FROM node:18-alpine AS builder
 
 WORKDIR /app
 
+# Accept build arguments
+ARG VITE_API_URL
+ARG VITE_UMAMI_URL
+ARG VITE_UMAMI_WEBSITE_ID
+
+# Set environment variables for build
+ENV VITE_API_URL=$VITE_API_URL
+ENV VITE_UMAMI_URL=$VITE_UMAMI_URL
+ENV VITE_UMAMI_WEBSITE_ID=$VITE_UMAMI_WEBSITE_ID
+
 # Copy package files
 COPY package*.json ./