fix: handle MetaMask Mobile network switch fallback

[sirakinb]

Fixes #215.

Summary

• Handle MetaMask Mobile's wrapped unknown-chain error (data.originalError.code === 4902) when switching networks.
• Await the wallet_addEthereumChain fallback so the button actually opens the add-network flow when Polygon Mumbai is missing.
• Show a wallet error message instead of failing silently, and disable the button while the wallet request is pending.

Testing

• Not run locally: this environment does not have npm/yarn/pnpm available and the fresh clone has no node_modules.

Notes

This keeps the existing HTTPS-RPC guard before attempting wallet_addEthereumChain.

[sirakinb]

Small CI note: the CLA Assistant check appears to be failing before it can post the usual signature prompt.

The check annotation says:

Could not retrieve repository contents. Status: 401

From .github/workflows/cla.yml, this workflow reads signatures from etherisc/cla-signatures via PERSONAL_ACCESS_TOKEN, so this looks like a CLA workflow credential/access issue rather than a code/test failure in this PR. Happy to complete the CLA step once the workflow can prompt/check it.

[sirakinb]

Maintainer note: the CLA Assistant check is failing before it can evaluate the PR. The workflow reports that it could not retrieve repository contents with status 401, which looks like a CLA Assistant token/app permission issue on the repository side rather than a code failure in this PR.