refactor: Reuse flutter local notifications object

[krille-chan]

Thank you so much for your contribution to FluffyChat ❤️❤️❤️

Please make sure that your Pull Request meet the following acceptance criteria:

• Code formatting and import sorting has been done with dart format lib/ test/ and dart run import_sorter:main --no-comments
• The commit message uses the format of Conventional Commits
• The commit message describes what has been changed, why it has been changed and how it has been changed
• Every new feature or change of the design/GUI is linked to an approved design proposal in an issue
• Every new feature in the app or the build system has a strategy how this will be tested and maintained from now on for every release, e.g. a volunteer who takes over maintainership

Pull Request has been tested on:

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