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

TODO.md

@@ -45,6 +45,13 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: The native token's label follows the active network
+  ([#372](https://git.eeqj.de/sneak/AutistMask/issues/372)). `networks.js` gives
+  each network a `nativeCurrency` and nothing read it: every screen wrote `ETH`,
+  so on Sepolia the balance, the value and the fee all read `ETH`. The balance
+  lists, Send, confirmation, approval, wait, success and error screens, and the
+  transaction history now read `nativeCurrency`, which is `ETH` on mainnet and
+  `SepoliaETH` on Sepolia.
 - 2026-10-04: A swap whose deadline is later than a JavaScript date can hold is
   decoded ([#437](https://git.eeqj.de/sneak/AutistMask/issues/437)). A date
   reaches only to 275760-09-13, so a later deadline, such as the `uint256`