feat: Improve database export with native sharing on mobile

- Enhance CapacitorPlatformService to use native share sheet via Web Share API
- Add fallback to Capacitor Share API with temporary file storage
- Implement WebPlatformService export with direct download for desktop
- Clean up temporary files and URLs after sharing/download

The changes provide a more platform-appropriate experience:
- Mobile: Uses system share sheet for flexible saving/sharing options
- Desktop: Direct download to user's download folder

src/services/PlatformService.ts

@@ -46,6 +46,15 @@ export interface PlatformService {
    */
   listFiles(directory: string): Promise<string[]>;
 
+  /**
+   * Exports a database blob to a file, handling platform-specific save operations.
+   * @param blob - The database blob to export
+   * @param fileName - The name of the file to save
+   * @returns Promise that resolves when the export is complete
+   * @throws Error if export fails
+   */
+  exportDatabase(blob: Blob, fileName: string): Promise<void>;
+
   // Camera operations
   /**
    * Activates the device camera to take a picture.