fix: say an unknown-scale balance the same way on Send and on confirm (closes #377)

When two addresses' explorer reports disagree on a token's decimals, the
Send screen showed the stored figure while the confirmation screen said
the balance was unknown. One function in send.js now gives both the
balance and scale, so both read `unknown (SYMBOL)`.

The confirmation screen's fee-unknown message names its cause: for an
unknown scale it says the wallet does not know the token's decimal
places and the transaction cannot be sent, instead of asking for a
retry that cannot help. Other causes keep the old sentence.

Model: opus-5-5

src/popup/views/confirmTx.js

@@ -198,6 +198,19 @@ function show(txInfo) {
     $("confirm-amount-fee-error").classList.toggle("hidden", isErc20);
     $("confirm-gas-error").classList.toggle("hidden", !isErc20);
 
+    // The fee-unknown message names its cause, which is also known here.
+    // Without the token's scale estimateGas() cannot encode the transfer, so
+    // the estimate fails every time and going back cannot help; any other
+    // failure may clear on a retry.
+    $("confirm-fee-unknown-error").textContent =
+        isErc20 && txInfo.tokenDecimals == null
+            ? "The network fee could not be estimated, because this wallet" +
+              " does not know how many decimal places this token uses, so" +
+              " this transaction cannot be sent."
+            : "The network fee could not be estimated, so this transaction" +
+              " cannot be checked against your balance. Please go back and" +
+              " try again.";
+
     renderValidation(txInfo);
 
     // Reset password field and error
@@ -244,7 +257,8 @@ function renderValidation(txInfo) {
     });
 
     // Messages carrying the user's own numbers are built here; the fixed
-    // sentences live in the reserved elements in index.html.
+    // sentences live in the reserved elements in index.html, except the
+    // fee-unknown one, which show() sets.
     const messages = [];
     if (codes.includes(CODES.AMOUNT_INVALID)) {
         messages.push("Please enter a valid amount to send.");