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.

src/types/global.d.ts

@@ -33,4 +33,37 @@ declare module '@jlongster/sql.js' {
   }) => Promise<SQL>;
 
   export default initSqlJs;
+}
+
+/**
+ * Electron API types for the main world context bridge.
+ * 
+ * These types define the secure IPC APIs exposed by the preload script
+ * to the renderer process for native Electron functionality.
+ */
+interface 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 with success status, file path, or error message
+   */
+  exportData: (fileName: string, data: string) => Promise<{
+    success: boolean;
+    path?: string;
+    error?: string;
+  }>;
+}
+
+/**
+ * Global window interface extension for Electron APIs.
+ * 
+ * This makes the electronAPI available on the window object
+ * in TypeScript without type errors.
+ */
+declare global {
+  interface Window {
+    electronAPI: ElectronAPI;
+  }
 }