docs: add VITE_API_BASE_URL documentation for remote/VPS deployment

[JiayuuWang]

Documentation Improvement

When deploying MiroFish on a VPS and accessing remotely, users get 'Network error' because:

1. The axios client in frontend/src/api/index.js defaults to http://localhost:5001
2. The Vite dev server proxy (/api → localhost:5001) only works in local development
3. When accessed remotely, the browser tries to connect to localhost:5001 on the browser's machine, not the VPS

Proposed Documentation

Document VITE_API_BASE_URL in .env.example and add remote deployment notes to README.

Files to Update

1. .env.example - Add VITE_API_BASE_URL with explanation
2. README.md - Add 'Remote/VPS Deployment Note' section
3. README-ZH.md - Add Chinese version of remote deployment note