docs(frontend): Fix proxy configuration example to match actual implementation

The README proxy configuration example incorrectly used process.env
instead of the env variable from loadEnv.

Changes:
- Update proxy target to use env.VITE_BACKEND_URL (not process.env)
- Ensure documentation matches vite.config.js implementation
- Maintain consistency across all Vite configuration examples

Addresses CodeRabbit critical feedback on PR #1607.

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Co-Authored-By: Claude <[email protected]>

Author: hari-kuriakose

frontend/README.md

@@ -185,7 +185,7 @@ Backend API calls are automatically proxied in development:
 server: {
   proxy: {
     '/api': {
-      target: process.env.VITE_BACKEND_URL,
+      target: env.VITE_BACKEND_URL,
       changeOrigin: true,
       secure: false,
     },