feat: add mock data generation option to menu for current day or week

[renjfk]

Add a menu option that allows users to populate the current day or week with mock data based on a selected sample date, accessible only through a hidden meta key combination for easier testing and simulation.

Details

• Add menu item in the popover that generates mock data for the current view (day or week)
• Menu item should only be visible when a meta key is pressed (e.g., Option key)
• Respect current view mode: generate data for current day if in day view, or current week if in week view
• Present date picker dialog to let user select which date to use as sample source
• Show destructive confirmation dialog warning that existing data will be overwritten
• Use existing MockDataGenerator.swift to populate target period with realistic data
• Sample data from the user-selected date to generate similar activities for each day
• Mock data should include both app usage and website usage sessions
• Provide visual feedback when mock data is generated (e.g., toast notification or brief message)
• Handle cases where selected sample date has no data available (show error message or disable generation)