Event edit modal shows times of timezone in which the event is created

### Problem

Today I have an event at 15:30, and in the sidebar, it shows up like that:


When I double-click the event, the times of the timezone in which the event was created are shown:


This is confusing.

### Proposal

### Option 1:
Show the event in the timezone the OS is set to.

### Option 2:
Make a dropdown in the timezone section to be able to switch between the timezone of the event or the timezone of the OS.

### Prior Art (Optional)

_No response_

Reproduce the discrepancy between the event shown in the sidebar and the event edit modal using an event created in another timezone. Start at those two UI entry points and determine the intended timezone behavior; done means the displayed times are consistent and the chosen timezone behavior is clear to users.

Written by the indexing model from the issue text.