Add single-instance enforcement to Electron app

- Implement app.requestSingleInstanceLock() to prevent multiple instances
- Add user-friendly dialog when second instance is attempted
- Focus and restore existing window instead of creating new instance
- Prevent database corruption and resource conflicts
- Update documentation with single-instance behavior details

electron/src/index.ts

@@ -1,7 +1,7 @@
 import type { CapacitorElectronConfig } from '@capacitor-community/electron';
 import { getCapacitorElectronConfig, setupElectronDeepLinking } from '@capacitor-community/electron';
 import type { MenuItemConstructorOptions } from 'electron';
-import { app, MenuItem, ipcMain } from 'electron';
+import { app, MenuItem, ipcMain, dialog } from 'electron';
 import electronIsDev from 'electron-is-dev';
 import unhandled from 'electron-unhandled';
 import { autoUpdater } from 'electron-updater';
@@ -10,6 +10,27 @@ import { join } from 'path';
 
 import { ElectronCapacitorApp, setupContentSecurityPolicy, setupReloadWatcher } from './setup';
 
+// Single instance enforcement
+const gotTheLock = app.requestSingleInstanceLock();
+
+if (!gotTheLock) {
+  console.log('[Electron] Another instance is already running. Exiting...');
+  
+  // Show user-friendly dialog before quitting
+  dialog.showMessageBox({
+    type: 'info',
+    title: 'TimeSafari Already Running',
+    message: 'TimeSafari is already running in another window.',
+    detail: 'Please use the existing TimeSafari window instead of opening a new one.',
+    buttons: ['OK']
+  }).then(() => {
+    app.quit();
+  });
+  
+  // Exit immediately
+  process.exit(0);
+}
+
 // Graceful handling of unhandled errors.
 unhandled({
   logger: (error) => {
@@ -79,6 +100,20 @@ autoUpdater.on('error', (error) => {
   // Initialize our app, build windows, and load content.
   await myCapacitorApp.init();
   
+  // Handle second instance launch (focus existing window)
+  app.on('second-instance', (event, commandLine, workingDirectory) => {
+    console.log('[Electron] Second instance attempted to launch');
+    
+    // Someone tried to run a second instance, we should focus our window instead
+    const mainWindow = myCapacitorApp.getMainWindow();
+    if (mainWindow) {
+      if (mainWindow.isMinimized()) {
+        mainWindow.restore();
+      }
+      mainWindow.focus();
+    }
+  });
+  
   // Only check for updates in production builds, not in development or AppImage
   if (!electronIsDev && !process.env.APPIMAGE) {
     try {