Fix tax-exempt customers always charged tax in quote preview

parseFloat('0') is falsy in JS, so '0 || pageMeta.taxPercent' was
falling through to the company default rate even when the TaxPercent
field was correctly set to 0 for a tax-exempt customer. Use an
explicit field presence check instead.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/wwwroot/js/item-wizard.js

@@ -2904,7 +2904,8 @@ async function runAutoPricing() {
     try {
         // Collect current form meta
         const customerId   = parseInt(document.querySelector('[name="CustomerId"]')?.value) || null;
-        const taxPercent   = parseFloat(document.querySelector('[name="TaxPercent"]')?.value) || pageMeta.taxPercent || 0;
+        const _taxField    = document.querySelector('[name="TaxPercent"]');
+        const taxPercent   = _taxField ? parseFloat(_taxField.value) : (pageMeta.taxPercent ?? 0);
         const discountType = document.getElementById('discountTypeSelect')?.value || 'None';
         const discountVal  = parseFloat(document.getElementById('discountValueInput')?.value) || 0;
         const isRushJob        = document.getElementById('IsRushJob')?.checked || false;