## Problem
`SD.begin()` return value was ignored. If the SD card fails to
initialize, the device continues and crashes when trying to load
settings/state.
## Fix
Check return value and display "SD card error" message instead of
proceeding with undefined state.
## Testing
- Builds successfully with `pio run`
- Affects: `src/main.cpp`
## Summary
* This PR drastically reshapes the structure of the codebase, moving
from the concept of "Screens" to "Activities", restructing the files and
setting up the concept of subactivities.
* This should help with keep the main file clean and containing all
functional logic in the relevant activity.
* CrossPointState is now also a global singleton which should help with
accessing it from within activities.
## Additional Context
* This is probably going to be a bit disruptive for people with open
PRs, sorry 😞
* white sleep screen
* quicker pwr button
* no extra spacing between paragraphs
* Added settings class with de/serialization and whiteSleepScreen setting to control inverting the sleep screen
* Added Settings screen for real, made settings a global singleton
* Added setting for extra paragraph spacing.
* fixed typo
* Rework after feedback
* Fixed type from bool to uint8
* First pass at moving to SDK EInkDisplay library
* Add 2-bit grayscale text and anti-aliased rendering of text
* Render status bar for empty chapters
* Refresh screen every 15 pages to avoid ghosting
* Simplify boot and sleep screens
* Give FileSelectionScreen task more stack memory
* Move text around slightly on Boot and Sleep screens
* Re-use existing buffer and write to whole screen for 'partial update'
