feat(export): add buffer download methods to ExportBuilder

- Added download() method to get statements as Buffer without saving to disk
- Added downloadAsArrayBuffer() method for web API compatibility
- Enhanced documentation for getAccountStatement() method
- Updated README with comprehensive examples
- No breaking changes, backward compatible

ts/bunq.classes.export.ts

@@ -334,4 +334,24 @@ export class ExportBuilder {
     await bunqExport.waitForCompletion();
     await bunqExport.saveToFile(filePath);
   }
+
+  /**
+   * Create and download export as Buffer
+   */
+  public async download(): Promise<Buffer> {
+    const bunqExport = await this.create();
+    await bunqExport.waitForCompletion();
+    return bunqExport.downloadContent();
+  }
+
+  /**
+   * Create and download export as ArrayBuffer
+   */
+  public async downloadAsArrayBuffer(): Promise<ArrayBuffer> {
+    const buffer = await this.download();
+    return buffer.buffer.slice(
+      buffer.byteOffset,
+      buffer.byteOffset + buffer.byteLength
+    );
+  }
 }