Add the Category builder to the tool menu

- [x] I have searched the [issues](https://github.com/ActivityWatch/activitywatch/issues) of this repo and believe that this is not a duplicate.
- [x] I have searched the [documentation](https://docs.activitywatch.net/en/latest/) and believe that my question is not covered.

## Issue

The categorization menu is a core part of the app. But it is hidden within the settings menu. Maybe the category builder can be put in the tools menu for easier access?


relevant code: https://github.com/ActivityWatch/aw-webui/blob/291da6f2c5e7a6b896f23a4eec5ffed9874321ba/src/components/Header.vue#L53-L60
```
b-navbar-nav.ml-auto
b-nav-item-dropdown
template(slot="button-content")
div.d-inline.px-2.px-lg-1
icon(name="tools")
| Tools
b-dropdown-item(to="/search")
icon(name="search")
| Search
```
Code to add:
```
b-dropdown-item(to="/settings/category-builder")
icon(name="INSERT ICON")
| Category builder
```