fix(security): remove innerHTML XSS sink in showAlert

templates/index.html

@@ -160,7 +160,26 @@
                  * @param {string} type - Bootstrap alert type (success, danger, warning, info)
                  */
                 showAlert(element, message, type = 'info') {
-                    element.innerHTML = `<div class="alert alert-${type}" role="alert">${message}</div>`;
+                    element.replaceChildren();
+                    const alert = document.createElement('div');
+                    alert.className = `alert alert-${type}`;
+                    alert.setAttribute('role', 'alert');
+                    if (typeof message === 'string') {
+                        alert.textContent = message;
+                    } else if (message && typeof message === 'object') {
+                        if (message.title) {
+                            const strong = document.createElement('strong');
+                            strong.textContent = message.title;
+                            alert.appendChild(strong);
+                        }
+                        if (message.body) {
+                            if (message.title) {
+                                alert.appendChild(document.createTextNode(' '));
+                            }
+                            alert.appendChild(document.createTextNode(message.body));
+                        }
+                    }
+                    element.appendChild(alert);
                 },
 
                 /**
@@ -465,7 +484,7 @@
                                 ui.showUserInfo(user);
                             }
                             
-                            utils.showAlert(elements.secretResult, `<strong>Secret Phrase:</strong> ${cachedSecret}`, 'success');
+                            utils.showAlert(elements.secretResult, { title: 'Secret Phrase:', body: cachedSecret }, 'success');
                         } else {
                             // Fetch fresh secret phrase and user info
                             setTimeout(ui.getSecretPhrase, 500);
@@ -581,7 +600,7 @@
                             localStorage.setItem('secret_phrase', data.secret_phrase);
                             localStorage.setItem('secret_last_fetch', Date.now().toString());
                             
-                            utils.showAlert(elements.secretResult, `<strong>Secret Phrase:</strong> ${data.secret_phrase}`, 'success');
+                                    utils.showAlert(elements.secretResult, { title: 'Secret Phrase:', body: data.secret_phrase }, 'success');
                         } else {
                             // Clear cached secret if user is no longer authorized
                             localStorage.removeItem('secret_phrase');
@@ -618,7 +637,7 @@
                             const price = data.bitcoin.chf;
                             utils.showAlert(
                                 elements.btcPriceResult,
-                                `<strong>1 Bitcoin (BTC) = CHF ${price.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2})}</strong>`, 
+                                { title: `1 Bitcoin (BTC) = CHF ${price.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2})}` },
                                 'success'
                             );
                         } else {