fix: improve file sharing on Android using app-private storage

- Replace direct file writing with app-private storage + share dialog
- Add Share plugin for cross-platform file sharing
- Update file paths configuration for Android
- Fix permission issues by using Directory.Data instead of Documents
- Simplify file export flow in DataExportSection

src/services/PlatformService.ts

@@ -57,6 +57,14 @@ export interface PlatformService {
    */
   writeFile(path: string, content: string): Promise<void>;
 
+  /**
+   * Writes content to a file at the specified path and shares it.
+   * @param fileName - The filename of the file to write
+   * @param content - The content to write to the file
+   * @returns Promise that resolves when the write is complete
+   */
+  writeAndShareFile(fileName: string, content: string): Promise<void>;
+
   /**
    * Deletes a file at the specified path.
    * @param path - The path to the file to delete