Add "Active Users" and "Returning Users" Metrics to Dashboard

Add two new user engagement metrics to the main dashboard:

• Active Users: users who performed any event in the selected time range

• Returning Users: users who were also active in the previous corresponding time range

Example logic:
For a 7-day range:

• Active Users = users with any event in the last 7 days

• Returning Users = users with events in both the last 7 days and the previous 7 days

Acceptance criteria:

• "Active Users" count displays correctly based on selected time range

• "Returning Users" count reflects users active in BOTH the current and previous time range of equal length

• Time ranges supported: 1 day, 7 days, 30 days, 3 month

• Data updates appropriately when the time range is changed