fix: approve-tx/approve-sign error divs consistency with confirm-tx

Add min-h-[1.25rem] and border styling to approve-tx-error and
approve-sign-error divs to prevent layout shift, matching the pattern
used by modal-password-error in confirm-tx view.

Replace direct DOM classList manipulation with showError()/hideError()
helpers from helpers.js for consistency.

Closes #84

src/popup/index.html

@@ -1139,7 +1139,10 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                     />
                 </div>
-                <div id="approve-tx-error" class="text-xs hidden mb-2"></div>
+                <div
+                    id="approve-tx-error"
+                    class="text-xs mb-2 border border-border border-dashed p-1 min-h-[1.25rem] hidden"
+                ></div>
                 <div class="flex justify-between">
                     <button
                         id="btn-approve-tx"
@@ -1202,7 +1205,10 @@
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
                     />
                 </div>
-                <div id="approve-sign-error" class="text-xs hidden mb-2"></div>
+                <div
+                    id="approve-sign-error"
+                    class="text-xs mb-2 border border-border border-dashed p-1 min-h-[1.25rem] hidden"
+                ></div>
                 <div class="flex justify-between">
                     <button
                         id="btn-approve-sign"