fix: Resolve frontend API connection issues in Docker

- Add setupProxy.js to proxy API calls to backend container
- Install http-proxy-middleware for development proxy support
- Update docker-compose.dev.yml to use backend service name
- Add HOST=0.0.0.0 to allow external connections to dev server
- Add empty favicon.ico to prevent 500 errors
- Update proxy configuration to use port 7510

This fixes the ERR_CONNECTION_REFUSED errors when the frontend
tries to connect to the backend API in Docker environment.

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

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

frontend/src/setupProxy.js

@@ -0,0 +1,16 @@
+const { createProxyMiddleware } = require('http-proxy-middleware');
+
+module.exports = function(app) {
+  app.use(
+    '/api',
+    createProxyMiddleware({
+      target: process.env.BACKEND_URL || 'http://backend:7510',
+      changeOrigin: true,
+      logLevel: 'debug',
+      onError: (err, req, res) => {
+        console.error('Proxy Error:', err);
+        res.status(500).json({ error: 'Proxy Error', message: err.message });
+      }
+    })
+  );
+};