fix(arc-testnet): correct native token symbol, decimals, and explorer URL

[Ccheh]

Three real chain-configuration bugs that affect any developer forking this sample:

1. lib/chain-config.ts — wrong native token symbol

```diff
export const NATIVE_TOKENS: Record<string, string> = {

• arcTestnet: "ARC",

• arcTestnet: "USDC",
  avalancheFuji: "AVAX",
  baseSepolia: "ETH",
  };
  ```

There is no "ARC" token. Arc's documented core property is USDC is the native gas token (see Circle skills use-arc/SKILL.md: "Arc is Circle's blockchain where USDC is the native gas token").

This value flows directly into three user-visible error messages in components/transfer-form.tsx (lines 79, 81, 83, 92, 94, 96, 97), telling users to acquire a non-existent "ARC" token to pay gas. A new user hitting the gas-insufficient path is directed to find "ARC" on Arc Testnet, which doesn't exist — they should be sent to the Circle Faucet to get USDC.

2. lib/circle/gateway-sdk.ts — wrong native decimals

```diff

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

• nativeCurrency: { name: 'USDC', symbol: 'USDC', decimals: 18 },
  ```

Arc's native gas token uses 18 decimals (like ETH on other EVM chains), not 6. ERC-20 USDC at 0x3600... has 6 decimals as a view, but the chain-level nativeCurrency.decimals in a viem/wagmi Chain definition refers to native gas, which is 18 on Arc. This is explicitly documented in use-arc/SKILL.md:

"Dual decimals: Native gas uses 18 decimals (like ETH on other chains). ERC-20 USDC uses 6 decimals. Mixing these up will produce incorrect amounts."

Empirical confirmation: the canonical Chain definition in circlefin/arc-commerce/lib/wagmi/config.ts uses decimals: 18. The current value of 6 causes viem/wagmi balance display to be off by a factor of 10^12 — a 1.0 USDC balance shows as 1,000,000,000,000 in any component that reads formatEther/formatUnits against this nativeCurrency.

3. lib/circle/gateway-sdk.ts — broken block explorer URL

```diff

• default: { name: 'Explorer', url: 'https://explorer.arc.testnet.circle.com' },

• default: { name: 'Arcscan', url: 'https://testnet.arcscan.app' },
  ```

explorer.arc.testnet.circle.com returns HTTP 000 (DNS does not resolve). The canonical Arc Testnet explorer is testnet.arcscan.app (referenced in circlefin/skills/use-arc/SKILL.md, circlefin/arc-fintech/lib/constants/block-explorers.ts, and circlefin/arc-commerce/components/wallet/network-indicator.tsx).

```bash
$ curl -sI -o /dev/null -w "%{http_code}\n" "https://explorer.arc.testnet.circle.com"
000
$ curl -sI -o /dev/null -w "%{http_code}\n" "https://testnet.arcscan.app"
200
```

Any "View on Explorer" link rendered from this chain config currently 404s.

---

These are the same kind of chain-config inheritance bugs circlefin/arc-fintech carries (filing a separate PR there). Fixing the sample apps prevents the bugs from propagating into every downstream fork that uses these as templates.

[Sertug17]

+1 verified all three fixes locally on a fresh clone:

1. decimals: 6 → decimals: 18 ✅
Arc Testnet uses USDC as native gas with 18 decimals, while ERC-20 USDC uses 6. With decimals: 6, any balance display via viem's formatEther/formatUnits defaults would be off by 10^12.

2. nativeCurrency.name: 'USD Coin' → 'USDC' ✅
Consistent with the symbol and matches Arc's docs.

3. Explorer URL ✅
Reproduced the issue:

$ curl -I https://explorer.arc.testnet.circle.com
HTTP 000 (connection failed)

$ curl -I https://testnet.arcscan.app
HTTP/2 200

4. chain-config.ts: arcTestnet: "ARC" → "USDC" ✅
Matches the native token symbol used in gateway-sdk.ts nice catch on the cross-file consistency.

Cross-repo context

The same explorer URL pattern was identified and is being fixed in arc-fintech#9 by @skyc1e. Worth checking whether arc-fintech's gateway-sdk.ts also has the decimals: 6 issue I haven't verified that yet, but if the chain config was copied across sample apps the same fix may apply there.

Nice clean PR 👍 hopefully gets a review soon.