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/index.html

@@ -698,15 +698,13 @@
                     You do not have enough ETH to pay the network fee for this
                     transfer. Please add ETH to this address and try again.
                 </div>
+                <!-- Its sentence names why the fee could not be estimated,
+                so show() in confirmTx.js sets it. -->
                 <div
                     id="confirm-fee-unknown-error"
                     class="mb-2 border border-border border-dashed p-2 text-xs"
                     style="visibility: hidden"
-                >
-                    The network fee could not be estimated, so this transaction
-                    cannot be checked against your balance. Please go back and
-                    try again.
-                </div>
+                ></div>
                 <div class="mb-2">
                     <label class="block mb-1 text-xs">Password</label>
                     <input