Add asset logo SVGs for XLM and USDC

[MJ-RWA]

Trivial complexity — 100 pts

Summary

AssetBadge currently displays text symbols (✦ for XLM, $ for USDC) inside colored circles. Using actual SVG logos for both assets makes the app look polished and recognizable.

Files to Modify

• Create: src/assets/xlm-logo.svg — Stellar's official diamond logo (white on transparent, optimized <5KB)
• Create: src/assets/usdc-logo.svg — Circle's USDC logo (optimized <5KB)
• Modify: src/components/AssetBadge.tsx — use <img src={logo} /> for known assets, fallback to first letter

Where to get the SVGs

• XLM: https://www.stellar.org/brand
• USDC: https://www.circle.com/en/usdc/brand-kit

Implementation Notes

In AssetBadge.tsx, create a assetLogos map and render an <img> (w-3/5 h-3/5 object-contain) when a logo exists, otherwise fall back to the existing letter span.

Acceptance Criteria

• XLM shows the official Stellar diamond logo
• USDC shows the official Circle USDC logo
• Logos render correctly in all three badge sizes (sm, md, lg)
• Unknown assets fall back to the first letter of the asset code
• SVG files are under 5KB each (optimized)

Screen Recording Requirements

1. Dashboard with XLM and USDC balance cards — logos displayed
2. Send page asset selector — logos displayed
3. Swap page from/to cards — logos displayed