Refactor Calendar's Code and Optimize UI

[othbcq]

Description:

This issue addresses the feedback provided by @Plouc314 in the recent pull request review. The goal is to improve the overall code structure, readability, and test coverage of the Calendar feature.

Tasks:

1. CalendarViewModel:

• Refactor handleRecurringEvents and handleNonRecurringEvents to improve readability and avoid side effects.
• Replace MutableStateList with StateFlow and use a constant for the ICAL_URL.
• Introduce an IcalRepository for easier implementation switching (e.g., HTTP, mock).

2. CalendarViewModel Tests (#27)

• Write unit tests for filtering and sorting events.
• Test the handling of recurring events, ensuring they are properly processed and displayed.
• Test multi-day event handling to confirm accurate representation across consecutive days.
• Ensure edge cases are covered (e.g., events spanning the start or end of the testing period).

3. UI Components:

• Combine EventListWithResults and EventList into a single modular component.
• Pass icalEvents directly to CalendarGrid instead of the full viewModel.
• Create a DateUtils module to group all date-related helper functions for reuse across files and to avoid duplication.
• Change the "Look Up Event" button to a magnifying glass icon for better clarity and usability. This will make the button more intuitive without disrupting the layout.