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

tests/coldWorkerChainSwitch.test.js

@@ -39,7 +39,7 @@ function walletFixture() {
         {
             name: "Wallet 1",
             type: "hd",
-            addresses: [{ address: ADDRESS, balance: "0", tokenBalances: [] }],
+            addresses: [{ address: ADDRESS, balance: null, tokenBalances: [] }],
         },
     ];
 }