fix: declare setupfee_off_with_symbol locally to avoid implicit global (#1247)

Add let declaration for setupfee_off_with_symbol inside the
applies_to_setup_fee block, mirroring the pattern used for off_with_symbol.
Without this, the variable was assigned without let/const/var at lines 172
and 174, creating an implicit global write.

Resolves #1237

Author: superdav42

assets/js/coupon-code.js

@@ -167,6 +167,7 @@
 							}
 
 							if (applies_to_setup_fee) {
+								let setupfee_off_with_symbol = '';
 
 								if (setup_fee_discount_type != '"absolute"') {
 									setupfee_off_with_symbol = ''.concat(setup_fee_discount_value, '%');