feat: Replace WP-Cron with Action Scheduler for reliable auto-sync

[HardeepAsrani]

Summary

PR Pro: https://github.com/Codeinwp/visualizer-pro/pull/573

Switches the scheduled data refresh hooks in both the free and pro plugins from WP-Cron to Action Scheduler, which is bundled via Composer.

Will affect visual aspect of the product

No

Screenshots

Test instructions

Prerequisites: Visualizer + Visualizer Pro installed with an existing sync schedule configured
WP-Crontrol installed

1. Migration from Existing Install
   • Confirm visualizer_schedule_refresh_db and visualizer_schedule_import exist in WP-Crontrol
   • Update both plugins to this build and visit any wp-admin page
   • Confirm both hooks are gone from WP-Crontrol
   • Confirm both appear in Tools → Scheduled Actions under the visualizer group with no duplicates
2. Sync Fires Correctly
   • In Tools → Scheduled Actions, find visualizer_schedule_import and click Run
   • Confirm the chart data updates and the next run is re-queued
3. WP-Cron Disabled
   • Add define( 'DISABLE_WP_CRON', true ); to wp-config.php
   • Trigger visualizer_schedule_import manually from Tools → Scheduled Actions
   • Confirm the chart still syncs successfully

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/visualizer-pro/issues/555.

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