Developer Activity Heatmap and Time Insights

Add a day-wise activity visualization to the profile page that shows when a developer is most active. The feature displays activity aggregated by day of the week and further broken down into configurable time slots. This helps surface personal work patterns such as peak coding days, preferred hours, and consistency across the week. Data is derived from commits, pull requests, and reviews and presented as an intuitive heatmap with weekly and monthly views.