chore: Follow strict dart style

[johannesvedder]

New Features

• Use universal lint package for all Flutter and Dart packages
• Line length reduced to default length of 80 instead of 120
• Revised configuration of json_serializable build process

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 704273f):

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• https://studyu-dev-designer--pr641-chore-default-dart-s-c37qu0cr.web.app

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