Add per-license customer secrets for API response verification

- Add static methods to ResponseSigner for deriving customer-specific secrets
- Display "API Verification Secret" in customer account licenses page
- Add collapsible secret section with copy button
- Update server-implementation.md with per-license secret documentation
- Update translations with new strings

Each customer now gets a unique verification secret derived from their
license key, eliminating the need to share the master server secret.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/js/frontend.js

@@ -19,6 +19,7 @@
 
         bindEvents: function() {
             $(document).on('click', '.copy-license-btn', this.copyLicenseKey);
+            $(document).on('click', '.copy-secret-btn', this.copySecret);
 
             // Transfer modal events
             $(document).on('click', '.wclp-transfer-btn', this.openTransferModal.bind(this));
@@ -28,6 +29,9 @@
             // Older versions toggle
             $(document).on('click', '.older-versions-toggle', this.toggleOlderVersions);
 
+            // Secret toggle
+            $(document).on('click', '.secret-toggle', this.toggleSecret);
+
             // Close modal on escape key
             $(document).on('keyup', function(e) {
                 if (e.key === 'Escape') {
@@ -50,6 +54,47 @@
             $list.slideToggle(200);
         },
 
+        /**
+         * Toggle secret visibility
+         */
+        toggleSecret: function(e) {
+            e.preventDefault();
+
+            var $btn = $(this);
+            var $content = $btn.siblings('.secret-content');
+            var isExpanded = $btn.attr('aria-expanded') === 'true';
+
+            $btn.attr('aria-expanded', !isExpanded);
+            $content.slideToggle(200);
+        },
+
+        /**
+         * Copy secret to clipboard
+         */
+        copySecret: function(e) {
+            e.preventDefault();
+
+            var $btn = $(this);
+            var secret = $btn.data('secret');
+
+            if (!secret) {
+                return;
+            }
+
+            // Use modern clipboard API if available
+            if (navigator.clipboard && navigator.clipboard.writeText) {
+                navigator.clipboard.writeText(secret)
+                    .then(function() {
+                        WCLicensedProductFrontend.showCopyFeedback($btn, true);
+                    })
+                    .catch(function() {
+                        WCLicensedProductFrontend.fallbackCopy(secret, $btn);
+                    });
+            } else {
+                WCLicensedProductFrontend.fallbackCopy(secret, $btn);
+            }
+        },
+
         /**
          * Copy license key to clipboard
          */