asset: Implement fallback logic for missing token icons

[AlAfiz]

Description

When we list new or unknown tokens, we might not have a custom logo image for them yet.
Right now, missing images show an ugly broken image icon in the browser.
We need a fallback mechanism that generates a clean initial-based logo if the image fails to load.

Requirements

• Update the TokenIcon.tsx component.
• Use the onError attribute on the HTML <img> tag to detect loading failures.
• If the image fails, swap the UI to a colored circle (div).
• Display the first letter of the token symbol inside the circle (e.g., "U" for UnknownToken).

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[Wils580]

@Wils580 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I've been on something like this and I believe it is the best option to work with

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