fix: the ETH line reads unknown at an address no refresh has read (closes #736)

An address's ETH balance is now null until a refresh has read it. That covers
a new address, a new wallet, addresses a wallet scan finds, and every address
after a network switch.

Its line reads `quantity unknown`, Home's balance `unknown (ETH)`, and Send
`Current balance: unknown (ETH)`. Max says the balance is unknown. Send is no
longer refused as zero. The address has no total, and DeleteAddress warns that
it may hold a balance.

A refresh also deletes `balanceNotFetched` only when it read every address in
the stored record.

Model: opus-5-5

src/shared/chainSwitch.js

@@ -49,7 +49,7 @@ function clearPopupCaches() {
 // follow runs first, and this switch is made after it.
 //
 // The save writes only the fields that differ from what this page last saved.
-// When this page last saved a balance of "0", clearing that balance is no
+// When this page last saved a balance of null, clearing that balance is no
 // change. So the switch also resets these fields in the stored record:
 //
 // - every address's balance and token balances;
@@ -100,7 +100,7 @@ async function onChainSwitch(newNetworkId) {
         stored.fraudContracts = [];
         for (const wallet of stored.wallets) {
             for (const addr of wallet.addresses) {
-                addr.balance = "0";
+                addr.balance = null;
                 addr.tokenBalances = [];
             }
         }