add: renewal notice banner

[nirbhayel]

✨ PR Description

Purpose: Add subscription renewal notification system to display time-sensitive banners alerting users about upcoming or expired Ally accessibility plugin subscriptions across WordPress admin pages.

Main changes:

• Implemented Renewal_Notice class with date-based logic to display warning/error banners 30 days before expiration, at expiration, and 7 days after
• Added utility methods is_wp_dashboard_page() and is_wp_settings_page() to control banner visibility across WordPress admin screens
• Created notice.css stylesheet with responsive warning/error banner styling including dismissible functionality with 24-hour per-user cooldown

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[pkniazevych]

Looks good.