[Bug] iconUri is constructed with new Uri(iconUri, UriKind.Relative)

I found that iconUri is constructed with new Uri(iconUri, UriKind.Relative), which rejects absolute URLs. Since the Unity Inspector doesn't mention this requirement, it's easy to configure an absolute URL and spend hours debugging why Phantom continues to show the default SDK branding (solana.unity-sdk.gg / Solana.Unity-SDK). Could the SDK either support absolute URLs or validate and log a clear error when an invalid iconUri is supplied?

Search the SDK for `new Uri(iconUri, UriKind.Relative)` and inspect how `iconUri` is supplied through the Unity Inspector. Reproduce the behavior with an absolute URL, then verify that absolute URLs work or that invalid values produce a clear validation error and log message.

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