fix(arc-testnet): correct nativeCurrency decimals from 6 to 18

[Sertug17]

Summary

lib/circle/gateway-sdk.ts:47 defines Arc Testnet's native currency as:

nativeCurrency: { name: 'USD Coin', symbol: 'USDC', decimals: 6 }

However, Arc Testnet uses USDC as native gas with 18 decimals, while ERC-20 USDC uses 6. Because viem's chain config treats nativeCurrency.decimals as the value used for formatEther / formatUnits defaults and balance calculations, balances displayed via this chain config will be off by 10^12.

Verification

The same chain config pattern was identified and corrected in arc-multichain-wallet#37 by @Ccheh, where the same fix was applied alongside additional corrections.

Change

-  nativeCurrency: { name: 'USD Coin', symbol: 'USDC', decimals: 6 },
+  nativeCurrency: { name: 'USDC', symbol: 'USDC', decimals: 18 },

The name field was also updated from 'USD Coin' to 'USDC' for consistency with the symbol field (matching the change made in arc-multichain-wallet#37).

Notes

• Single-line change (1 file, +1/-1).
• This PR is independent of #9 by @skyc1e, which addresses the explorer URL inconsistency on a different line in the same file.
• The chain config appears to be copied across sample apps; same fix should be checked in any other apps using this template.

[Sertug17]

Closing this in favor of #19 by @Ccheh — I missed that PR while
reviewing the repo, my apologies for the duplication.

#19 covers the same decimals: 6 → 18 fix on the same line,
plus the broken explorer URL on the next line (explorer.arc.testnet.circle.com
→ testnet.arcscan.app) — it's the more complete PR and
should be the one merged.

Apologies @Ccheh — your PR is the right one.