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

networks.js gives each network a nativeCurrency (ETH, SepoliaETH) and nothing
read it: every screen wrote a hardcoded ETH, so on Sepolia the balance, the
value and the fee all read ETH. A nativeCurrency() helper in the popup's
helpers.js returns the active network's, and every native balance, value, fee,
reserve, type line and history entry is labelled with it. The "ETH" that
selectedToken and txInfo.token hold is the native token's id and is unchanged.
The confirmation screen's not-enough-to-pay-the-fee sentence now names the
coin to add, so show() sets it.

Model: opus-5-5

README.md

@@ -846,6 +846,14 @@ wherever shown. If a formatting rule applies in one place, it applies in every
 place. Users should never see the same value rendered differently on two
 screens.
 
+The native token's label is the active network's `nativeCurrency` in
+`src/shared/networks.js`: `ETH` on mainnet, `SepoliaETH` on Sepolia. Wherever
+this document shows ETH as the label of a native balance, value or fee, or in a
+"Native ETH transfer" type line, Sepolia shows `SepoliaETH`. The swap lines keep
+`ETH`, the router's own name for the native currency, and the ETH/USD price
+line, the contract-recipient warning and the refusal of a fee above 1 ETH keep
+their fixed wording.
+
 **Specific Exception — Truncation:** On some non-critical display locations, we
 may truncate _a small number_ of characters from the middle of an address solely
 due to display size constraints. Wherever possible, and, notably, **in all