Add self-update feature via GitHub Releases

Checks for updates on startup (silent, badge on button if available)
and on manual click. Downloads new portable .exe to temp dir, creates
a batch script to swap the running executable after quit, then relaunches.

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

styles.css

@@ -185,7 +185,9 @@ body {
 
 .content-header {
     margin-bottom: 24px;
-    text-align: center;
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
 }
 
 .content-header h1 {
@@ -424,6 +426,187 @@ button:disabled {
     animation: pulse 2s infinite;
 }
 
+/* Update Button */
+.btn-update {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    background: transparent;
+    color: var(--text-secondary);
+    border: 1px solid var(--button-outline);
+    padding: 6px 12px;
+    font-size: 12px;
+    font-weight: bold;
+    border-radius: 4px;
+    cursor: pointer;
+    transition: all 0.2s ease;
+    white-space: nowrap;
+    position: relative;
+}
+
+.btn-update:hover:not(:disabled) {
+    color: var(--text-primary);
+    border-color: var(--accent-primary);
+}
+
+.btn-update:hover:not(:disabled) .update-icon {
+    animation: spin 0.6s ease;
+}
+
+.btn-update:disabled .update-icon {
+    animation: spin 1s linear infinite;
+}
+
+.btn-update.update-available {
+    color: var(--success);
+    border-color: var(--success);
+    box-shadow: 0 0 8px rgba(76, 175, 80, 0.3);
+}
+
+.btn-update.update-available::after {
+    content: '';
+    position: absolute;
+    top: -3px;
+    right: -3px;
+    width: 8px;
+    height: 8px;
+    background: var(--success);
+    border-radius: 50%;
+    box-shadow: 0 0 6px rgba(76, 175, 80, 0.8);
+}
+
+.update-icon {
+    font-size: 14px;
+    display: inline-block;
+}
+
+@keyframes spin {
+    from { transform: rotate(0deg); }
+    to { transform: rotate(360deg); }
+}
+
+/* Update Modal */
+.update-modal-overlay {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 100%;
+    height: 100%;
+    background: rgba(0, 0, 0, 0.6);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    z-index: 1000;
+}
+
+.update-modal-card {
+    background: var(--card-bg);
+    border: 1px solid var(--border);
+    border-radius: 8px;
+    width: 480px;
+    max-width: 90%;
+    max-height: 80vh;
+    display: flex;
+    flex-direction: column;
+    box-shadow: 0 8px 32px rgba(0, 0, 0, 0.4);
+}
+
+.update-modal-header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: 16px 20px;
+    border-bottom: 1px solid var(--border);
+}
+
+.update-modal-header h3 {
+    font-size: 16px;
+    color: var(--text-primary);
+    margin: 0;
+}
+
+.update-modal-close {
+    background: transparent;
+    border: none;
+    color: var(--text-secondary);
+    font-size: 20px;
+    cursor: pointer;
+    padding: 0 4px;
+    line-height: 1;
+}
+
+.update-modal-close:hover {
+    color: var(--text-primary);
+}
+
+.update-modal-body {
+    padding: 20px;
+    overflow-y: auto;
+    flex: 1;
+}
+
+.update-modal-message {
+    font-size: 14px;
+    color: var(--text-primary);
+    margin: 0 0 12px 0;
+}
+
+.update-modal-notes {
+    font-size: 13px;
+    color: var(--text-secondary);
+    line-height: 1.5;
+    white-space: pre-wrap;
+    max-height: 200px;
+    overflow-y: auto;
+    padding: 12px;
+    background: var(--bg-primary);
+    border-radius: 4px;
+    border: 1px solid var(--border);
+}
+
+.update-modal-notes:empty {
+    display: none;
+}
+
+.update-progress-container {
+    margin-top: 16px;
+    display: flex;
+    align-items: center;
+    gap: 12px;
+}
+
+.update-progress-track {
+    flex: 1;
+    height: 8px;
+    background: var(--bg-primary);
+    border-radius: 4px;
+    overflow: hidden;
+    border: 1px solid var(--border);
+}
+
+.update-progress-fill {
+    height: 100%;
+    background: var(--accent-primary);
+    border-radius: 4px;
+    transition: width 0.3s ease;
+}
+
+.update-progress-text {
+    font-size: 12px;
+    font-weight: bold;
+    color: var(--text-secondary);
+    min-width: 36px;
+    text-align: right;
+}
+
+.update-modal-footer {
+    display: flex;
+    justify-content: flex-end;
+    gap: 8px;
+    padding: 16px 20px;
+    border-top: 1px solid var(--border);
+}
+
 /* Responsive Design */
 @media (max-width: 768px) {
     .devices-sidebar {