fix: a tracked token reads quantity unknown at an address no refresh has read (closes #720)

A new address, a new wallet and a network switch each leave tracked tokens
with no balance entry, so their lines read 0.0000 before any refresh had read
them.

Each of these now sets `balanceNotFetched` on the tracked tokens, and a refresh
already clears it. Their lines read `quantity unknown` until then. Addresses
that the wallet scan finds get the same. A refresh that did not read every
address the page now holds leaves the field set.

Model: opus-5-5

src/popup/views/send.js

@@ -242,10 +242,11 @@ function updateSendBalance() {
             state.trackedTokens,
             currentNetwork().chainId,
         );
-        // A null balance is a holding whose scale is unknown. Saying a figure
-        // for it would be a claim about the amount, so it reads as the
-        // confirmation screen's balance line reads it; the send itself is
-        // refused later by transferAmountUnits() for the same missing scale.
+        // A null balance is a holding whose scale is unknown, or a tracked
+        // token whose balance no refresh has read. Saying a figure for it would
+        // be a claim about the amount, so it reads as the confirmation screen's
+        // balance line reads it. transferAmountUnits() later refuses a send at
+        // a missing scale.
         const bal = tokenBalanceAndDecimals(addr, token).tokenBalance;
         $("send-balance").textContent =
             bal == null