feat(platform): implement dual-flow file sharing with Save to Device option

Add new platform service methods for direct file saving alongside existing share functionality:
- Add saveToDevice() and saveAs() methods to PlatformService interface
- Implement cross-platform support in WebPlatformService, ElectronPlatformService, and CapacitorPlatformService
- Update DataExportSection UI to provide Share Contacts and Save to Device buttons
- Add AndroidFileSaver plugin architecture with fallback implementation
- Include comprehensive documentation for native Android plugin implementation

This addresses the Android simulator file sharing limitation by providing users with clear choices between app-to-app sharing and direct device storage, while maintaining backward compatibility across all platforms.

- CapacitorPlatformService: Add MediaStore/SAF support with graceful fallbacks
- UI Components: Replace single download button with dual-action interface
- Documentation: Add AndroidFileSaver plugin implementation guide
- Type Safety: Maintain interface consistency across all platform services

src/services/platforms/WebPlatformService.ts

@@ -97,9 +97,7 @@ export class WebPlatformService implements PlatformService {
         }
       } else {
         // We're in a worker context - skip initBackend call
-        // Use console for critical startup message to avoid circular dependency
-        // eslint-disable-next-line no-console
-        console.log(
+        logger.info(
           "[WebPlatformService] Skipping initBackend call in worker context",
         );
       }
@@ -594,6 +592,48 @@ export class WebPlatformService implements PlatformService {
     }
   }
 
+  /**
+   * Saves content directly to the device's Downloads folder (web platform).
+   * Uses the browser's download mechanism to save files.
+   *
+   * @param fileName - The filename of the file to save
+   * @param content - The content to write to the file
+   * @returns Promise that resolves when the file is saved
+   */
+  async saveToDevice(fileName: string, content: string): Promise<void> {
+    try {
+      // Web platform: Use the same download mechanism as writeAndShareFile
+      await this.writeAndShareFile(fileName, content);
+      logger.log("[WebPlatformService] File saved to device:", fileName);
+    } catch (error) {
+      logger.error("[WebPlatformService] Error saving file to device:", error);
+      throw new Error(
+        `Failed to save file to device: ${error instanceof Error ? error.message : "Unknown error"}`,
+      );
+    }
+  }
+
+  /**
+   * Opens the system file picker to let the user choose where to save a file (web platform).
+   * Uses the browser's download mechanism with a suggested filename.
+   *
+   * @param fileName - The suggested filename for the file
+   * @param content - The content to write to the file
+   * @returns Promise that resolves when the file is saved
+   */
+  async saveAs(fileName: string, content: string): Promise<void> {
+    try {
+      // Web platform: Use the same download mechanism as writeAndShareFile
+      await this.writeAndShareFile(fileName, content);
+      logger.log("[WebPlatformService] File saved as:", fileName);
+    } catch (error) {
+      logger.error("[WebPlatformService] Error saving file as:", error);
+      throw new Error(
+        `Failed to save file as: ${error instanceof Error ? error.message : "Unknown error"}`,
+      );
+    }
+  }
+
   /**
    * @see PlatformService.dbQuery
    */