fix(frontend): use relative baseURL in production, avoid hardcoded localhost

[JasonOA888]

Problem

Frontend baseURL defaults to http://localhost:5001 in all environments, causing:

1. Users can only access from the machine running MiroFish
2. Docker deployments with non-5001 ports fail
3. Server deployments require frontend rebuild to change URL

Solution

Make baseURL environment-aware:

• Production: Use relative path (empty string) for same-origin deployment
• Development: Keep localhost:5001 default
• Override: VITE_API_BASE_URL still takes highest priority

const getBaseURL = () => {
  if (import.meta.env.VITE_API_BASE_URL) {
    return import.meta.env.VITE_API_BASE_URL
  }
  if (import.meta.env.PROD) {
    return ''  // relative path in production
  }
  return 'http://localhost:5001'  // dev default
}

Testing

• Dev: Works with localhost:5001
• Prod (Docker): Works with any port mapping
• Custom: Works with VITE_API_BASE_URL set

Fixes #93, related to #59, #57