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/deleteAddress.js

@@ -87,15 +87,22 @@ function recoveryPathText(wallet) {
 // when every holding here is one with no price, since "$0.00" directly under
 // "This address holds a balance." is a contradiction. A token holding below
 // 0.000001 does not count, as the lines below leave it out.
+//
+// While no refresh has read the ETH balance the address may be empty, so the
+// sentence says "may hold".
 function balanceWarningHtml(addr) {
     if (!addressHoldsFunds(addr)) return " ";
     const line = formatAddressTotal(getAddressValue(addr));
     const total = line
         ? `<div class="text-xs text-muted mt-1">${escapeHtml(line)}</div>`
         : "";
+    const sentence =
+        addr.balance == null
+            ? "This address may hold a balance."
+            : "This address holds a balance.";
     return (
-        `<p class="mb-1">This address holds a balance. Removing it does not ` +
-        `move or spend anything; the balance stays at the address.</p>` +
+        `<p class="mb-1">${sentence} Removing it does not move or spend ` +
+        `anything; the balance stays at the address.</p>` +
         balanceLinesForAddress(addr, state.trackedTokens, false) +
         total
     );