catalog: replace placeholder GUID with genuine random GUID

[CCoupel]

Context

Emby plugin catalog uses the plugin GUID as the unique identity for updates and installation. Using a placeholder or non-unique GUID will conflict with other plugins in the catalog.

Problem

`Plugin.cs` currently declares a hardcoded placeholder GUID. This must be replaced with a genuine random GUID before catalog submission.

Fix

1. Generate a new random GUID once
2. Replace the placeholder in `Plugin.cs`:
   ```csharp
   public override Guid Id => new Guid("xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx");
   ```
3. This GUID must never change once the plugin is published — it is the permanent identity

Priority

Critical — catalog submission will be rejected or conflict without this fix.