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/shared/prices.js

@@ -146,7 +146,8 @@ function formatUsd(amount) {
 // way an absent holders_count is not a count of zero.
 //
 //   usd:     the value of the holdings a price is known for, or null when
-//            nothing is knowable at all — testnet, or before the first fetch.
+//            no total can be stated — testnet, before the first fetch, or an
+//            ETH balance no refresh has read.
 //   partial: the address also holds a token with no price, so usd is a floor
 //            and not the total.
 //
@@ -155,7 +156,8 @@ function getAddressValue(addr) {
     const { currentNetwork } = require("./state");
     if (currentNetwork().isTestnet) return { usd: null, partial: false };
     if (!prices.ETH) return { usd: null, partial: false };
-    let usd = parseFloat(addr.balance || "0") * prices.ETH;
+    if (addr.balance == null) return { usd: null, partial: false };
+    let usd = parseFloat(addr.balance) * prices.ETH;
     let partial = false;
     for (const token of addr.tokenBalances || []) {
         // A holding below 0.000001 is left out, as the balance lists leave it