chore: Refactor main UI to use CustomScrollView and Slivers

[BlakeBarrett]

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As a Lore user, I want the app to feel exactly like a Google produced Material app, so that I feel confident using it.

Notes:

Use these widgets:

• https://api.flutter.dev/flutter/widgets/CustomScrollView-class.html
• https://api.flutter.dev/flutter/material/SliverAppBar-class.html
• https://api.flutter.dev/flutter/material/FlexibleSpaceBar-class.html