Add coupons customers generation to Settings page

[masteradhoc]

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• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

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Changes proposed in this Pull Request:

Customers and coupons can currently only be generated via WP-CLI (wc generate customers / wc generate coupons). This PR exposes both generators in the admin UI, on par with products and orders.

Changes are limited to includes/Admin/Settings.php:

• Added DEFAULT_NUM_CUSTOMERS and DEFAULT_NUM_COUPONS constants (default: 10)
• Added "Generate customers" and "Generate coupons" form sections to the settings page
• Added the corresponding process_page_submit() branches that call BatchProcessor::create_new_job()

No other files required changes — Router.php and BatchProcessor.php already supported both slugs. The date-range Advanced Options are intentionally not passed to customers/coupons, as neither generator uses them.

Closes # .

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How to test the changes in this Pull Request:

1. Navigate to Tools → Smooth Generator in wp-admin.
2. Verify "Generate customers" and "Generate coupons" sections appear with a number input and a Generate button each.
3. Enter a number, click Generate, and confirm the progress bar runs and the correct number of customers/coupons is created in WooCommerce.

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Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

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Changelog entry

Expose customer and coupon generation in the admin UI. Both generators were previously available via WP-CLI only; they can now be triggered directly from the Smooth Generator settings page.

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FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.