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>

frontend/next.config.js

@@ -28,10 +28,34 @@ const nextConfig = {
         hostname: '192.168.0.202',
         pathname: '/uploads/**',
       },
+      {
+        protocol: 'https',
+        hostname: 'staging.autonetsellcar.com',
+        pathname: '/uploads/**',
+      },
+      {
+        protocol: 'https',
+        hostname: 'autonetsellcar.com',
+        pathname: '/uploads/**',
+      },
     ],
   },
   env: {
-    NEXT_PUBLIC_API_URL: process.env.NEXT_PUBLIC_API_URL || 'http://192.168.0.202:8000',
+    // Don't override empty string - it means use relative paths with rewrites
+    NEXT_PUBLIC_API_URL: process.env.NEXT_PUBLIC_API_URL,
+  },
+  async rewrites() {
+    const backendUrl = process.env.BACKEND_URL || 'http://localhost:8000';
+    return [
+      {
+        source: '/api/:path*',
+        destination: `${backendUrl}/api/:path*`,
+      },
+      {
+        source: '/uploads/:path*',
+        destination: `${backendUrl}/uploads/:path*`,
+      },
+    ];
   },
 }