## Problem solved
When supported source chains is updated, update `fromChain` to take the first chain on the supported sources list instead of defaulting to `polygon`
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## PR-Codex overview
The focus of this PR is to update the default source chain selection in `PayEmbed`.
### Detailed summary
- Updated default source chain selection logic in `BuyScreen.tsx`
- Refactored the dependencies array in `useEffect` hook for better performance
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