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>

preload.js

@@ -19,5 +19,19 @@ contextBridge.exposeInMainWorld('electronAPI', {
         console.error('Extension cookie handler error:', error);
       }
     });
+  },
+
+  // Self-update functionality
+  checkForUpdates: () => ipcRenderer.invoke('check-for-updates'),
+  downloadAndInstallUpdate: (params) => ipcRenderer.invoke('download-and-install-update', params),
+  getCurrentVersion: () => ipcRenderer.invoke('get-current-version'),
+  onUpdateDownloadProgress: (callback) => {
+    ipcRenderer.on('update-download-progress', (event, data) => {
+      try {
+        callback(data);
+      } catch (error) {
+        console.error('Update download progress handler error:', error);
+      }
+    });
   }
 });