[Simple Analytics] Add Data Layer & Form Event Tracking

[alaca]

Description of the Change

• Created a custom database table ({prefix}_mailchimp_sf_form_analytics) for daily aggregated form analytics, with columns for list_id, form_id, event_date, views, and submissions (unique key on list_id, form_id, event_date)
• Implement client-side form view tracking that adds data-list-id attribute to all form types (block, widget, shortcode) and
  fires an AJAX POST on page load, deduplicated per list_id
• Hook into successful form submissions via a new mailchimp_sf_form_submission_success action to increment daily submission
• Table creation runs on plugin activation and via the upgrade routine for existing installs

Closes #198

How to test the Change

• Activate the plugin on a fresh install and verify the wp_mailchimp_sf_form_analytics table is created
• Visit a page with a Mailchimp signup form and verify a view is recorded in the analytics table for the correct list_id and
  today's date
• Place multiple forms (different lists) on one page and verify each list gets its own view count
• Submit a form successfully and verify the submission count increments
• Submit a form with invalid data (e.g., missing email) and verify no submission is tracked
• Navigate to Mailchimp > Analytics in wp-admin and verify that test data is displayed

Changelog Entry

Created a custom database table (mailchimp_sf_form_analytics) for daily aggregated form analytics
Implemented client-side form view tracking via AJAX

Credits

Props @alaca

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.