feat: improved UX with custom events

- For custom events that span multiple days, when changing the colour, could we have a pop-up that says ‘Would you like to change it for all?’ similar to Apple
- potentially have options to handle these custom events individually

Start by locating the custom-event colour-change flow and the UI entry point for events spanning multiple days. Confirm how the current flow treats a multi-day event, then define the popup choices and verify that changing the colour either applies to all related events or only the selected occurrence.

Written by the indexing model from the issue text.