Help protect dapps against wrong-network confusion

As non-mainnet uses increase, we will more often see dapps accidentally suggesting transactions on the wrong network.

We should provide safety mechanisms to help prevent this scenario.

## Simple proposed solution

- Add optional `chain_id` parameter to all key-related methods.
- If this parameter is missing from a transaction, _warn the user_ to verify the network (Nudge dapps to always include this parameter)
- Document this recommended parameter in our docs.
- If this parameter is different than the current selected network, return an informative response/error to the dapp, for suggesting network switching.
- Coordinate the change between products.

Start by reviewing the proposed chain_id behavior for key-related methods and the selected-network transaction flow; no file or test is named. Define the cross-product API contract, warning or error behavior, documentation changes, and completion criteria before implementation.

Written by the indexing model from the issue text.