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/chainSwitchGate.test.js

@@ -328,7 +328,7 @@ describe("only the user switches the network", () => {
         const after = bg.walletState();
         expect(after.networkId).toBe("sepolia");
         expect(after.rpcUrl).toBe(SEPOLIA.defaultRpcUrl);
-        expect(after.wallets[0].addresses[0].balance).toBe("0");
+        expect(after.wallets[0].addresses[0].balance).toBeNull();
         expect(bg.chainChangedEvents()).toEqual([
             {
                 type: "AUTISTMASK_EVENT",