Use existing Flask app as the local server inside the desktop app
This removes some duplicated code. It doesn't appear to improve performance at all.
It also means that you can once again use the planalyzer in a web browser. It's a little fiddly, but you run a command like this:
python3 ~/.local/lib/python3.7/site-packages/planalyze/backend/localserver.py ~/.local/share/planalyzer/frontend/ ~/.cache/planalyze/plans.json ~/.cache/planalyze/logs.json