Feat: Categorising events

- allowing custom events to be grouped/tagged so that when you change the colour of one, it changes the colour of all in the same group
- potentially like github issue labels where users can create different categories and tag a block to one of those categories so that these events become linked

Start by tracing the existing custom event creation flow and how event colours are represented. Determine where category or group data would belong, then define the user flow for creating categories, tagging events, and applying a colour change across a group; the feature is done when those linked events consistently share the selected colour.

Written by the indexing model from the issue text.