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/popupFollowsNetworkSwitch.test.js

@@ -134,7 +134,7 @@ test.each([
     await backgroundSwitch(page, "sepolia");
 
     expect(page.visibleViews()).toEqual(["main"]);
-    expect(page.text("total-value")).toBe("0.0000 SepoliaETH");
+    expect(page.text("total-value")).toBe("unknown (SepoliaETH)");
 });
 
 // A request on Ethereum Mainnet fails, so Home's list holds the failure message.