I don't want to do a 'look at my tool, it's better' comment (it's not better!) - but per your request for feedback, I just wanted to mention a (sort of) similar project I've been recently working on: https://git.mig5.net/mig5/bouquin, and it might give you some ideas for taking yours further:

Some similarities:

- 'one note per day' design

- days with notes have a visual 'marker' in the calendar

- full encryption at rest

But there are some big differences:

- It's a Qt GUI app (not web based, no syncing)

- It does a lot of other things (document storage, time logging, invoicing etc)

I mention it only because I implemented a few things based on feedback for my own app, that I could see would be useful in yours! Those suggestions are:

- Live markdown syntax (I couldn't use # or * or other Markdown symbols in your app, and that's a pretty popular option, for formatting)

- Version control of notes (in Bouquin, every change auto-saves and there is a sort of 'git diff' version interface that allows reverting to an earlier version)

- Tagging and search

- Perhaps support images

- Code blocks (this is kind of the same as the markdown idea, but syntax highlighting would be a huge win)

One other rather obscure bit of feedback I got for my app, was for people who use it as a 'TODO' planner: for items that were left 'unchecked' (incomplete) at the end of the day, I managed to make it move those items automatically to the next day, to help carry across unfinished work. I was sceptical at first, but it surprisingly useful once implemented!

Anyway, well done! Looking forward to seeing what comes next!