Improve importing data from different sources

[simonwep]

Right now you can only

• Import a previously exported CSV file from googles annual budget planner.
• Import a previously exported json file.

It'd be nice to have a template that can be download and imported, preferable in CSV.

Discussed in #107

^{Originally posted by bennypowers May 13, 2025}
So I've got a big old backup from toshl.com I'd like to import, which columns should I use, etc?