Highlight Rescheduled Personal Events with a Red Border

Is your feature request related to a problem? Please describe.

Description:
I would like to propose a feature enhancement for Nextcloud Calendar that visually highlights personal events that have been rescheduled relative to their original time, particularly when linked to a main event in a shared calendar.

Currently, when a personal event linked to a main event in a shared calendar is rescheduled (e.g., time or date changes), there is no clear visual indication that the event has moved from its original slot. This can lead to confusion, especially when managing multiple events or coordinating with others. Notably, modifying a personal event does not affect the main event in the shared calendar, as described in issue #5708.

For example:

• A user has a personal reminder for a team meeting scheduled at 10:00 AM (linked to the main event in the shared calendar).
• The user moves their personal reminder to 11:30 AM for personal convenience.
• There should be an immediate visual indication that the personal event has been moved from its original time.

Describe the solution you'd like

Introduce a visual indicator (such as a red border) around personal events that have been rescheduled relative to their original time.

✅ Highlight rescheduled personal events with a red border in the calendar view.
✅ Provide an optional tooltip displaying the original time/date for reference.
✅ Allow users to manually acknowledge the change, which would remove the highlight.
✅ Ensure the feature applies to personal events linked to shared events without altering the shared event itself.

Describe alternatives you've considered

• Track the original scheduled time of a personal event when it is created.
• Compare the original time with the current time to determine if a change has occurred.
• Apply a red border (or another distinct indicator) to visually highlight the modification.
• Provide an option to acknowledge or dismiss the highlight.

Additional context

This feature would improve the user experience by offering a clear, visual way to track event modifications. It would help prevent missed or overlooked changes and enhance productivity for Nextcloud Calendar users.

Thank you for your consideration! 🚀

Start by reviewing issue #5708 and the calendar view implementation to determine how personal events link to shared events and retain their original time. No specific files or tests are named in this issue. Done would include a red-border indicator, an optional original-time tooltip, acknowledgement behavior, and no changes to the shared event.