[Simple Analytics] Add Analytics Admin Page, Navigation & Controls #197

Description

Add a new "Analytics" submenu page under Mailchimp with page, CSS, JS scaffolding, date range selector (presets + custom date selection), list filter dropdown, and a deep link to Mailchimp analytics.

Technical Context:

• Admin menus registered in Mailchimp_Admin::add_admin_menu_pages().
• Existing settings page is mailchimp_sf_options via add_menu_page()
• Templates are in includes/admin/templates
• Available lists are in mailchimp_sf_lists option, currently selected list in mc_list_id.

Implementation Breakdown

1. Admin Page Registration

• Create includes/class-mailchimp-analytics.php with init(), register_admin_page(), render_page(), enqueue_scripts() methods
• Register submenu via add_submenu_page() under mailchimp_sf_options with slug mailchimp_sf_analytics, capability manage_options
• Only register page if Mailchimp account is connected.
• Create template includes/admin/templates/analytics.php with page header matching existing Mailchimp admin header pattern

2. Date Range Selector

• Preset dropdown: Last 7 days, Last 30 days (default), Last 90 days, Last 6 months, Last year, Custom
• "Custom" reveals from/to date pickers.
• Display resolved date range text (e.g., "Jan 1, 2026 - Jan 30, 2026")

3. List Filter Dropdown

• Populated from mailchimp_sf_lists option, defaults to mc_list_id
• Changing triggers data refresh for all sections.

4. Deep Link & Styling

• Add "View detailed analytics in Mailchimp" link to https://DATA_CENTER.admin.mailchimp.com/analytics/audience-analytics/ (opens in new tab)
• Create assets/css/analytics.css
• Create assets/js/analytics.js
• Enqueue Chart.js + analytics assets only on analytics page.

Acceptance Criteria

• "Analytics" and "Form Settings" appear as submenu items under Mailchimp
• The “Analytics” page will only be added if a Mailchimp account is connected.
• Date range dropdown works with presets and custom calendar picker
• List filter shows all connected lists and defaults correctly
• Changing filters triggers section refresh
• Deep link opens Mailchimp dashboard in new tab with rel="noopener noreferrer"
• Assets are only loaded on the analytics page
• Page requires manage_options capability