When running as a PyInstaller bundle, __file__ resolves to a temp
extraction dir (/tmp/_MEIxxxxx/) that is deleted on exit, so all
saved credentials and config were silently lost.

Fix: introduce DATA_DIR = ~/.auto_note/ when sys.frozen is set,
= PROJECT_DIR otherwise (source runs unchanged). All credential and
config file paths (config.json, *_token.txt, *_api.txt, manifest.json,
course data dirs) now use DATA_DIR.