added PageStorageKey to categories list to maintain its scroll position

[ggurdin]

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:

• [ ] Android
• [ ] iOS
• [ ] Browser (Chromium based)
• [ ] Browser (Firefox based)
• [ ] Browser (WebKit based)
• [ ] Desktop Linux
• [ ] Desktop Windows
• [ ] Desktop macOS