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/send.js

@@ -230,11 +230,14 @@ function updateSendBalance() {
     attachCopyHandlers($("send-from"));
     const token = state.selectedToken || $("send-token").value;
     if (token === "ETH") {
+        // A null balance is one no refresh has read.
         $("send-balance").textContent =
-            "Current balance: " +
-            truncateAmountNeverZero(addr.balance || "0") +
-            " " +
-            nativeCurrency();
+            addr.balance == null
+                ? "Current balance: unknown (" + nativeCurrency() + ")"
+                : "Current balance: " +
+                  truncateAmountNeverZero(addr.balance) +
+                  " " +
+                  nativeCurrency();
     } else {
         const symbol = resolveSymbol(
             token,
@@ -289,6 +292,12 @@ async function fillMaxAmount() {
         return;
     }
 
+    // A null balance is one no refresh has read.
+    if (addr.balance == null) {
+        showFlash("This token's balance is unknown.");
+        return;
+    }
+
     // The fee is estimated for this recipient, as the confirmation screen
     // estimates it: sending to a contract can cost more gas.
     const to = $("send-to").value.trim();
@@ -306,7 +315,7 @@ async function fillMaxAmount() {
             provider.estimateGas({
                 from: addr.address,
                 to,
-                value: parseEther(addr.balance || "0"),
+                value: parseEther(addr.balance),
             }),
         ]);
         feeWei = feeReserveWei(gasLimit, feeData);
@@ -321,8 +330,10 @@ async function fillMaxAmount() {
     }
     // While the estimate was in flight the user left the screen (and perhaps
     // opened it again), typed an amount, or changed the address, the holding
-    // or the recipient: what they did wins.
+    // or the recipient: what they did wins. A network switch in that time
+    // leaves no balance to fill in from.
     if (
+        addr.balance == null ||
         state.currentView !== "send" ||
         sendScreenOpenings !== opening ||
         currentAddress()?.address !== addr.address ||