Support "Material You" dynamic colours

[nikclayton]

Now that the app is (mostly) converted over to using Material 3 design tokens it should be relatively straightforward to follow https://m3.material.io/styles/color/dynamic-color/overview and implement support for dynamic colour.

When complete this should give the user two additional theme options in the preferences:

1. A colour scheme generated from the user's wallpaper (the docs call "User-generated color")
2. A colour scheme generated from an image the user provides (the docs call "Color from content")

A couple of options to consider for option (2) above.

a. The user can specify an arbitrary image, those colours are used across the app, or b. The user can have the colour scheme generated from their account's icon or header image, giving them a per-account colour scheme in the app.

[nikclayton]

Note to self: the branches of the fork at https://github.com/rimar1337/pachli-android have some Material You changes.