fix: default wallet button components to mainnet chainscan

[yqrashawn]

Summary

• Changed default props in OKXButton and MetaMaskButton components from Galileo testnet (chain ID 16602, chainscan-galileo.0g.ai) to mainnet (chain ID 16661, chainscan.0g.ai)
• Updated testnet-overview.md to explicitly pass testnet chain params to wallet buttons, since those buttons now default to mainnet

Context

This is part of a broader SEO initiative to ensure chainscan.0g.ai (mainnet) ranks above testnet URLs in search results. Having the components default to testnet caused any doc page that embedded wallet buttons without explicit props to silently reference the testnet block explorer.

Related: 0g-chain-scan-website #11 (parent chainscan SEO fix)
Slack thread: https://0g-labs.slack.com/archives/C0ADTBNJH7F/p1773723361528969

Changes

File	Change
src/components/OKXButton/index.tsx	Default chainId → 16661, chainName → "0G Mainnet", rpcUrls → evmrpc.0g.ai, blockExplorerUrls → chainscan.0g.ai/
src/components/MetaMaskButton/index.tsx	Same defaults updated to mainnet
docs/developer-hub/testnet/testnet-overview.md	Wallet buttons now explicitly pass all Galileo testnet props so they continue to add the correct testnet network

No Breakage

• mainnet-overview.md already explicitly passes all mainnet props — no change needed there
• testnet-overview.md previously relied on defaults (which happened to be testnet) — now passes explicit testnet props, same behavior preserved

[vercel]

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[yqrashawn]

Closing this PR — the component default change was unnecessary and caused issues.

Analysis: All references in are already in correct testnet contexts:

• → testnet chainscan ✓
• → both testnet and mainnet sections correctly labeled ✓
• → testnet config section ✓

No mainnet doc page incorrectly links to testnet chainscan, so there's nothing to fix here for SEO.

Bug introduced: Changing defaults to mainnet also removed the state and trigger — breaking the Newton→Galileo migration flow for testnet users.

The SEO fix is already covered by:

• — per-environment titles, meta descriptions, canonical tags, and robots meta
• — storagescan internal link fixed