Add Stellar asset selector to bounty creation

[nicbstme]

Summary

• add a reusable AssetSelector for bounty payout assets
• include XLM, Stellar USDC, EURC, and custom Stellar issuer entry
• validate custom issuer addresses with Zod and wire the selector into the create bounty form plus existing scaffold

Verification

• npm run build
• Browser check on /en/bounties/create: opened dropdown, selected Add Custom Asset, confirmed invalid issuer shows validation and a valid 56-character issuer clears it

Closes #215