fix(wagmi): classify Arc Testnet as testnet and update canonical testnet USDC addresses

[vhtgzzl]

Summary

This PR fixes two observable issues in the Wagmi network and USDC configuration:

1. Fix network selector categorization for Arc Testnet (useNetworkSupport.ts & config.ts):

   • Previously, the network classification filter checked only /sepolia|amoy/, which caused the repo's primary demo network, Arc Testnet, to be incorrectly categorized and listed under MAINNETS in the wallet network selector modal.
   • Added testnet: true to the arcTestnet chain definition and updated the categorization helper isTestnetChain to check c.testnet as well as testnet name regex patterns.

2. Update canonical Circle testnet USDC contract addresses (usdcAddresses.ts):

   • Replaced outdated placeholder addresses on Sepolia, Optimism Sepolia, and Polygon Amoy with official Circle-deployed testnet USDC contracts (0x1c7... on Sepolia, 0x5fd... on Optimism Sepolia, 0x41E... on Polygon Amoy).
   • This ensures USDC balance checks and checkout approval flows work seamlessly on these testnet networks.

Testing & Verification

• Verified Arc Testnet is correctly categorized under TESTNET_CHAINS.
• Verified canonical contract addresses match official Circle developer documentation and CCTP deployments.
• Clean diff with no breaking changes to mainnet configurations.