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/css/frontend.css

@@ -863,3 +863,118 @@
     color: #2271b1;
     font-weight: 500;
 }
+
+/* Customer Secret Section */
+.license-row-secret {
+    margin-top: 0.75em;
+    padding-top: 0.75em;
+    border-top: 1px dashed #e5e5e5;
+}
+
+.secret-toggle {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.35em;
+    padding: 0.4em 0.75em;
+    background: transparent;
+    border: 1px solid #ddd;
+    border-radius: 4px;
+    font-size: 0.85em;
+    color: #666;
+    cursor: pointer;
+    transition: all 0.2s ease;
+}
+
+.secret-toggle:hover {
+    background: #f5f5f5;
+    border-color: #ccc;
+    color: #333;
+}
+
+.secret-toggle .dashicons {
+    font-size: 14px;
+    width: 14px;
+    height: 14px;
+}
+
+.secret-toggle .toggle-arrow {
+    transition: transform 0.2s ease;
+}
+
+.secret-toggle[aria-expanded="true"] .toggle-arrow {
+    transform: rotate(180deg);
+}
+
+.secret-content {
+    margin-top: 0.75em;
+    padding: 1em;
+    background: #f8f9fa;
+    border-radius: 4px;
+    border: 1px solid #e5e5e5;
+}
+
+.secret-description {
+    margin: 0 0 0.75em 0;
+    font-size: 0.85em;
+    color: #666;
+}
+
+.secret-value-wrapper {
+    display: flex;
+    align-items: center;
+    gap: 0.5em;
+}
+
+.secret-value {
+    font-family: 'SF Mono', Monaco, Consolas, monospace;
+    font-size: 0.75em;
+    background: #fff;
+    padding: 0.5em 0.75em;
+    border: 1px solid #ddd;
+    border-radius: 4px;
+    word-break: break-all;
+    flex: 1;
+    min-width: 0;
+    overflow-x: auto;
+}
+
+.copy-secret-btn {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    width: 32px;
+    height: 32px;
+    padding: 0;
+    background: #f0f0f0;
+    border: 1px solid #ddd;
+    border-radius: 4px;
+    cursor: pointer;
+    transition: all 0.2s ease;
+    flex-shrink: 0;
+}
+
+.copy-secret-btn:hover {
+    background: #e5e5e5;
+    border-color: #ccc;
+}
+
+.copy-secret-btn .dashicons {
+    font-size: 18px;
+    width: 18px;
+    height: 18px;
+}
+
+@media screen and (max-width: 768px) {
+    .secret-value-wrapper {
+        flex-direction: column;
+        align-items: stretch;
+    }
+
+    .secret-value {
+        font-size: 0.7em;
+    }
+
+    .copy-secret-btn {
+        align-self: flex-start;
+    }
+}