Add scheme failsafe to all inventory URL link buttons

If a stored URL is missing http:// or https:// the browser treats it as relative
and appends it to the app URL. Guard in three places:

- inventory-catalog-lookup.js syncLinkButton: ensureAbsoluteUrl() prepends https://
- inventory-label-scan.js syncLink: same guard for scan-filled URL fields
- Details.cshtml SafeUrl() Razor helper on SpecPageUrl, SdsUrl, TdsUrl links

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

src/PowderCoating.Web/wwwroot/js/inventory-catalog-lookup.js

@@ -413,10 +413,15 @@
 
     // ── Helpers ───────────────────────────────────────────────────────────────
 
+    function ensureAbsoluteUrl(url) {
+        if (!url) return url;
+        return /^https?:\/\//i.test(url) ? url : 'https://' + url;
+    }
+
     function syncLinkButton(inputId, linkId, url) {
         const link = document.getElementById(linkId);
         if (!link) return;
-        if (url) { link.href = url; link.classList.remove('d-none'); }
+        if (url) { link.href = ensureAbsoluteUrl(url); link.classList.remove('d-none'); }
         else      { link.classList.add('d-none'); }
     }