feat: Set up a database and contact persistence.

[robinlinden]

Not sure if we want to check in the generated code or not. It seems to be 50/50 what people do in the Flutter dev community.

I currently have this PR set up with all generated files ignored, with scripts to generate them under tools/. We'll need some form of preparation scripts there even if we check in the generated files as different builds need different things downloaded. E.g. the web build needs the wasm/js files from 3rd-party libraries, and Windows needs sqlite3.dll.

See: https://stackoverflow.com/questions/56110386/should-i-commit-generated-code-in-flutter-dart-to-vcs https://pub.dev/packages/build_runner#source-control

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[robinlinden]

You can view the database used on the preview page in the Firefox dev tools here: