Material 3 support

[hunting4code]

Describe the bug
This project currently uses Material 2 which has been set as deprecated by flutter but may have a seperate package (See flutter/flutter#143499).

[clragon]

We might not want to migrate to Material 3, which clashes heavily with our current design, and would prompt having to redesign various parts of the app. It also generally is more difficult to apply brand identity, even in small parts like primary colors, in Material 3.

If allowed by the deprecation timeline, we could wait until Flutter has decoupled material and cupertino design (see here: flutter/flutter#101479) and then pick one of the emerging Widget frameworks for this app, to construct both a stable and sustainable as well as recognizable UI.