fix: Use current host URL instead of hardcoded localhost for API calls

[StuartF303]

Changed frontend to use dynamic base URL instead of hardcoded localhost:5000.

Problem:

• Frontend was hardcoded to use http://localhost:5000 for API calls
• In production on Fly.io, this caused requests to fail (localhost in browser)
• Browser tried accessing http://localhost:5000/api/tanks instead of the deployed URL

Solution:

• Use builder.HostEnvironment.BaseAddress for API base URL
• This automatically uses the current host:
  • Production (Fly.io): https://vandaemon.fly.dev
  • Local dev: http://localhost:8080 (or whatever port serves the app)
• Nginx proxies /api and /hubs requests to the backend API (port 5000)

Result:

• API calls now work correctly: https://vandaemon.fly.dev/api/tanks
• SignalR connects to: https://vandaemon.fly.dev/hubs/telemetry
• Works in both production and local development environments