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/services/platforms/WebPlatformService.ts

@@ -559,13 +559,39 @@ export class WebPlatformService implements PlatformService {
   }
 
   /**
-   * Not supported in web platform.
-   * @param _fileName - Unused fileName parameter
-   * @param _content - Unused content parameter
-   * @throws Error indicating file system access is not available
+   * Downloads a file in the web platform using blob URLs and download links.
+   * Creates a temporary download link and triggers the browser's download mechanism.
+   * @param fileName - The name of the file to download
+   * @param content - The content to write to the file
+   * @returns Promise that resolves when the download is initiated
    */
-  async writeAndShareFile(_fileName: string, _content: string): Promise<void> {
-    throw new Error("File system access not available in web platform");
+  async writeAndShareFile(fileName: string, content: string): Promise<void> {
+    try {
+      // Create a blob with the content
+      const blob = new Blob([content], { type: "application/json" });
+
+      // Create a temporary download link
+      const url = URL.createObjectURL(blob);
+      const downloadLink = document.createElement("a");
+      downloadLink.href = url;
+      downloadLink.download = fileName;
+      downloadLink.style.display = "none";
+
+      // Add to DOM, click, and remove
+      document.body.appendChild(downloadLink);
+      downloadLink.click();
+      document.body.removeChild(downloadLink);
+
+      // Clean up the object URL after a short delay
+      setTimeout(() => URL.revokeObjectURL(url), 1000);
+
+      logger.log("[WebPlatformService] File download initiated:", fileName);
+    } catch (error) {
+      logger.error("[WebPlatformService] Error downloading file:", error);
+      throw new Error(
+        `Failed to download file: ${error instanceof Error ? error.message : "Unknown error"}`,
+      );
+    }
   }
 
   /**