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>

src/Frontend/AccountController.php

@@ -9,6 +9,7 @@ declare(strict_types=1);
 
 namespace Jeremias\WcLicensedProduct\Frontend;
 
+use Jeremias\WcLicensedProduct\Api\ResponseSigner;
 use Jeremias\WcLicensedProduct\License\LicenseManager;
 use Jeremias\WcLicensedProduct\Product\VersionManager;
 use Twig\Environment;
@@ -114,6 +115,7 @@ final class AccountController
             echo $this->twig->render('frontend/licenses.html.twig', [
                 'packages' => $packages,
                 'has_packages' => !empty($packages),
+                'signing_enabled' => ResponseSigner::isSigningEnabled(),
             ]);
         } catch (\Exception $e) {
             // Fallback to PHP template if Twig fails
@@ -161,6 +163,7 @@ final class AccountController
                 'status' => $license->getStatus(),
                 'expires_at' => $license->getExpiresAt(),
                 'is_transferable' => in_array($license->getStatus(), ['active', 'inactive'], true),
+                'customer_secret' => ResponseSigner::getCustomerSecretForLicense($license->getLicenseKey()),
             ];
 
             // Track if package has at least one active license