fix: TxApproval links addresses to the explorer of the transaction's network (closes #631)

TxApproval built its From, Contract and decoded-action address links for
the active network, so a Sepolia transaction reviewed with mainnet active
linked to the mainnet explorer. approvalAddressHtml() now takes the
transaction's chain id and passes it to renderAddressHtml(), as SuccessTx
and ErrorTx do. A chain id that names no network has no explorer, so its
address lines are drawn without a link and the screen still shows. SignApproval
and SiteApproval still link for the active network. The walletDefects
fixture's numeric chain id, which names no network, is now the hex string
the background sends.

Model: opus-5-5

README.md

@@ -2395,6 +2395,8 @@ view would leave a wallet one click from deletion.
     - Raw data: full calldata displayed inline (shown if present)
     - Password input and an error line
     - "Confirm" / "Reject" buttons
+- **Behavior**: Every etherscan link on the screen opens the explorer of the
+  network the transaction was populated on, whichever network is active.
 - **Transitions**:
     - "Confirm" (correct password) → decrypts and signs the transaction it was
       shown, exactly as shown, hands the signed transaction to the background to