[web-app-notes] Make ToC loading cancellable

[kulmann]

Initial Checklist

• I understand this is a feature request and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

Idea

In packages/web-app-notes/src/composables/useLoadToc.ts loading the table of contents should be done in a cancellable task. Otherwise we'll finish to load the ToC even when leaving the app.