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/backgroundStateIsolation.test.js

@@ -445,7 +445,7 @@ describe("a balance refresh under a network switch", () => {
 
         const after = bg.persisted();
         expect(after.networkId).toBe("mainnet");
-        expect(after.wallets[0].addresses[0].balance).toBe("0");
+        expect(after.wallets[0].addresses[0].balance).toBeNull();
         expect(after.lastBalanceRefresh).toBe(0);
 
         await bg.fireBalanceAlarm();