the web companion, web application, is spin-off made by @foxmask
You can go either way, improve or create a new one whatever is best in this situation.
- I would like to point out that Nextcloud is heavily using vue. Hence there are already components available (side bar, app navigation etc) that you could then use right away. This will then directly look an feel like all of the other Nextcloud apps. So this might be something to consider
- Fetching the notes can be done writing a small customer API should not be hard.
a simple web search
nextcloud api authentication - Qwant Search got me to User Authentication and Group Assignment with External REST API - dev / app dev - Nextcloud community so I found User Provisioning API — Nextcloud 11 Server Administration Manual 11 alpha documentation and eventually I ended up at Client APIs — Nextcloud latest Developer Manual latest documentation what is properly what your are looking for.
- it is a trade off between the frameworks and it is up to you if you want to create a standalone app. like the web application or improve the current one to uses the API of the groupware such as nextcloud to communicate with them or you are going to write nextcloud app.
- you can also think about how to improve the backend as there has to be a server what hopefully going to allow to query multiple profiles as you don't want to run a server per profile (think of families)
- another thing you can think about, how to make the notes available to the search engine of the groupware
read also here