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>

index.html

@@ -35,6 +35,10 @@
             <main class="main-content">
                 <div class="content-header">
                     <h1>Alta Video Camera Proxy</h1>
+                    <button type="button" id="checkUpdateBtn" class="btn-update" title="Check for Updates">
+                        <span class="update-icon">&#x21bb;</span>
+                        <span class="update-text">Check for Updates</span>
+                    </button>
                 </div>
 
                 <!-- API Connection Section -->
@@ -77,6 +81,30 @@
         </div>
     </div>
 
+    <!-- Update Modal -->
+    <div id="updateModalOverlay" class="update-modal-overlay" style="display: none;">
+        <div class="update-modal-card">
+            <div class="update-modal-header">
+                <h3>Update Available</h3>
+                <button type="button" id="updateModalCloseBtn" class="update-modal-close">&times;</button>
+            </div>
+            <div class="update-modal-body">
+                <p id="updateModalMessage" class="update-modal-message"></p>
+                <div id="updateModalNotes" class="update-modal-notes"></div>
+                <div id="updateProgressContainer" class="update-progress-container" style="display: none;">
+                    <div class="update-progress-track">
+                        <div id="updateProgressFill" class="update-progress-fill" style="width: 0%"></div>
+                    </div>
+                    <span id="updateProgressText" class="update-progress-text">0%</span>
+                </div>
+            </div>
+            <div class="update-modal-footer">
+                <button type="button" id="updateInstallBtn" class="btn-primary">Install Update</button>
+                <button type="button" id="updateLaterBtn" class="btn-outline">Later</button>
+            </div>
+        </div>
+    </div>
+
     <script src="renderer.js"></script>
 </body>
 </html>