Set up top level navigation

[zsmb13]

Fixes #236

This PR updates the navigation setup of the app the following way:

• Most screens are kept at the top level as before, within the root nav controller
• The onboarding flow "start screens" are in their own little nested nav graph
  • For now, there's a fake Flow that produces a random true/false value for whether the onboarding has been completed, we'll hook this up to actual preference storage later. This means the onboarding flow is skipped 50% of the time.
• The new HomeScreen component serves the screen with 4-button bottom navigation between its destinations. This can't be a nested nav graph as there's also common UI that persists through the screens (the bottom navigation bar), so it's a destination that contains a second NavHost that navigates within the HomeScreen destinations.
  • The Compose version bumps are needed for this nesting to work, as the new versions include this bugfix

[zsmb13]

Adding some basic insets here just to make buttons clickable, we'll need to do proper insets work later