Implement Simple Analytics

[vikrampm1]

Bring simple analytics directly into the plugin, allowing users to monitor sucess without leaving WP. Show a handful of graphs/metrics within a new admin page of the plugin to track success of the forms

More details and acceptance criteria: https://www.notion.so/fueled/Full-PRD-Mailchimp-for-WordPress-Simple-Analytics-3214dccd9d8b80429090c2022cb7d08c

https://www.figma.com/proto/JmK177E7mWxn3glDnPefoB/Mailchimp-2026-Projects?page-id=1%3A256&node-id=253-2012&viewport=73%2C312%2C0.26&t=AI2e7OL3jTKeJojE-8&scaling=scale-down&content-scaling=fixed

[ankitguptaindia]

@iamdharmesh

QA feedback Added: #212 (comment)

cc: @vikrampm1