NUI: fetch / RegisterNUICallback not responding

The problem

Your NUI (HTML/JS) does a fetch to a Lua callback but it never arrives, or returns 404, and the interface doesn't react.

The cause

The fetch URL uses an incorrect resource name. It must be exactly the resource name, which in JS is obtained with `GetParentResourceName()`.

The solution

Use the real resource name in the fetch and register the callback in Lua:

// NUI (JavaScript)
fetch(`https://${GetParentResourceName()}/miCallback`, {
  method: 'POST',
  headers: { 'Content-Type': 'application/json; charset=UTF-8' },
  body: JSON.stringify({ accion: 'comprar', id: 3 }),
}).then(r => r.json()).then(data => console.log(data));

Step by step

1. 1.In JS use `GetParentResourceName()` (don't hard-code the name).
2. 2.In Lua register the callback: `RegisterNUICallback('miCallback', function(data, cb) ... cb('ok') end)`.
3. 3.ALWAYS validate on the server whatever comes from the NUI; the client only requests.

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