Fix Mixed Content issue on staging (HTTPS/HTTP mismatch)

- Add Next.js API rewrites to proxy /api and /uploads requests
- Update docker-compose.staging.yml to use relative API paths
- Set NEXT_PUBLIC_API_URL to empty for staging (use rewrites)
- Add BACKEND_URL env var for Next.js server-side proxying
- Update all files to use relative paths when API_URL is empty

This fixes the issue where the staging site (HTTPS) was trying to
load resources from HTTP backend, causing Mixed Content errors.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.staging.yml

@@ -4,12 +4,13 @@ services:
       context: ./frontend
       dockerfile: Dockerfile
       args:
-        - NEXT_PUBLIC_API_URL=http://192.168.0.202:8001
+        - NEXT_PUBLIC_API_URL=
     container_name: autonet-frontend-staging
     ports:
       - "3001:3000"
     environment:
       - NODE_ENV=staging
+      - BACKEND_URL=http://backend-staging:8000
     depends_on:
       - backend-staging
     restart: unless-stopped