Added onboarding activation range filters

I introduced selectable time windows (7 days, 30 days, 90 days, and all time) for the onboarding activation analytics in the admin dashboard. This lets us easily switch the view to analyze activation trends over different periods, making it more flexible to track progress and spot changes.

Implemented activation rate trend line

The onboarding activation chart now includes a trend line showing the activation rate over time, calculated as the ratio of feature activations to onboarding completions. This visual helps quickly understand how activation is progressing relative to completed onboardings.

Switched to completion-based activation summary

The summary metrics now focus on users who completed onboarding rather than just started it, providing a more accurate picture of activation rates and user engagement after onboarding finishes.

Other improvements

• Updated targeted tests to cover the new range filters and activation rate calculations.
• Refined date handling utilities to support the new all-time range and ensure consistent UTC day calculations.
• Improved documentation with a dedicated plan file and changelog entry outlining the feature and troubleshooting tips.