feat: filter timetable nav and warn on export

[condyl]

Closes #35, #77

This pull request refactors the calendar view and timetable filtering logic to improve how visible timetables are determined and displayed based on the current calendar view range. It removes the old duration change and pinning logic, introduces a new approach for filtering and deduplicating timetables by visible date range, and simplifies related state management throughout the calendar component.

Timetable Filtering and Calendar View Improvements:

• Added viewRange state and a handleDatesSet callback to CalendarComponent.jsx to track the currently visible calendar date range and trigger updates when the view changes.
• Introduced getVisibleTimetables and related helper functions to filter and deduplicate timetables based on the visible date range, replacing the previous duration-based logic. Now, only unique timetables relevant to the visible range are shown.
• Updated all relevant usages in CalendarComponent.jsx to use visibleTimetables instead of the full timetables array, ensuring the UI and navigation only operate on filtered timetables.

Removal of Old Duration/Pinning Logic:

• Removed handleDurationChange, previousDuration, and aprioriDurationTimetable logic from both the calendar view and timetable management hooks, as this is now handled by the new visible timetable logic.

FullCalendar Integration:

• Passed the new handleDatesSet callback to the FullCalendar config to update the view range whenever the calendar view changes.

These changes make the calendar more responsive to the current view, ensure only relevant and unique timetable options are presented, and remove complex, error-prone state related to duration and component pinning.