One thing that I miss though is a nice web interface for calendar and contacts. Especially if I want to do some sort of cleanup of data. Is this functionality planned? Or is there some open source web calendaring software that you would recommend?
I've also been looking for such a solution for years, and except for the short-lived Flock by the Open Whisper Systems people, I couldn't find anything. That's why I finally decided to create it almost a year ago.
Edit: you just edited your reply while I was writing mine, to answer your additional question:
For a variety of reasons, the browser is not a safe environment for such an application, at least not currently. There is however a CalDAV/CardDAV gateway (beta) that you can host locally and essentially use any application that supports those to quickly edit your data, or alternatively, if you are a programmer, there's an open source (like everything from EteSync) python API (both low-level and high-level) that you can use to manipulate the data.