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/popup/views/addressDetail.js

@@ -273,7 +273,8 @@ function init(_ctx) {
             state.wallets[state.selectedWallet].addresses[
                 state.selectedAddress
             ];
-        if (!addr.balance || parseFloat(addr.balance) === 0) {
+        // A null balance is one no refresh has read, so it is not refused.
+        if (parseFloat(addr.balance) === 0) {
             showFlash("Cannot send \u2014 zero balance.");
             return;
         }