chore: the native token's label follows the network (closes #372)

networks.js gives each network a nativeCurrency (ETH, SepoliaETH) and nothing
read it: every screen wrote ETH. The wallet's balances and the Send and
confirmation screens now use the active network's. A transaction's figures use
the network its chain id names, through nativeCurrencyByChainId(): the
approval value and fee, the wait, success and error screens, history entries,
the detail screen and the fee-limit refusal, so a site switching networks
cannot make one read as another network's coin. The "ETH" that selectedToken
and txInfo.token hold is the native token's id and is unchanged. A token
reporting any network's nativeCurrency is a spoof, and the detail screen calls
an entry a token transfer when it has a token contract.

Model: opus-5-5

src/shared/networks.js

@@ -76,6 +76,15 @@ function networkByChainId(chainId) {
     return null;
 }
 
+// The native currency of the network with this chain id. A transaction's
+// value and fee are labelled with the one of the chain the transaction is on,
+// which need not be the active network. `ETH` when the chain id is missing or
+// no network here has it.
+function nativeCurrencyByChainId(chainId) {
+    const network = networkByChainId(chainId);
+    return network ? network.nativeCurrency : "ETH";
+}
+
 // Build a block explorer link for the given path type and value.
 // type: "address" | "tx" | "token" | "block"
 function explorerLink(network, type, value) {
@@ -89,5 +98,6 @@ module.exports = {
     isKnownNetworkId,
     networkById,
     networkByChainId,
+    nativeCurrencyByChainId,
     explorerLink,
 };