feat: implement secure IPC-based file export for Electron

Replace sandboxed Capacitor filesystem with native IPC for reliable file exports:
- Add IPC handler in main process for direct Downloads folder access
- Expose secure electronAPI via contextBridge in preload script
- Update ElectronPlatformService to use native IPC with web fallback
- Add TypeScript definitions for electron APIs
- Fix file export issues where files were trapped in virtual filesystem
- Enable proper date-stamped backup filenames in Downloads folder
- Follow Electron security best practices with process isolation

Files now export directly to ~/Downloads with exact path feedback.

electron/src/preload.ts

@@ -1,4 +1,34 @@
+import { contextBridge, ipcRenderer } from 'electron';
+
 require('./rt/electron-rt');
 //////////////////////////////
 // User Defined Preload scripts below
 console.log('User Preload!');
+
+/**
+ * Expose secure IPC APIs to the renderer process.
+ * 
+ * This creates a bridge between the sandboxed renderer and the main process,
+ * allowing secure file operations while maintaining Electron's security model.
+ */
+contextBridge.exposeInMainWorld('electronAPI', {
+  /**
+   * Export data to the user's Downloads folder.
+   * 
+   * @param fileName - The name of the file to save (e.g., 'backup-2025-07-06.json')
+   * @param data - The content to write to the file (string)
+   * @returns Promise<{success: boolean, path?: string, error?: string}>
+   * 
+   * @example
+   * ```typescript
+   * const result = await window.electronAPI.exportData('my-backup.json', JSON.stringify(data));
+   * if (result.success) {
+   *   console.log('File saved to:', result.path);
+   * } else {
+   *   console.error('Export failed:', result.error);
+   * }
+   * ```
+   */
+  exportData: (fileName: string, data: string) => 
+    ipcRenderer.invoke('export-data-to-downloads', fileName, data)
+});