## Problem
Reading progress.bin used `SD.exists()` then `SD.open()` without
checking if open succeeded. Race conditions or SD errors could cause
file handle to be invalid.
## Fix
- Removed redundant `SD.exists()` check
- Check if file opened successfully before reading
- Verify correct number of bytes were read
## Testing
- Builds successfully with `pio run`
- Affects: `src/activities/reader/EpubReaderActivity.cpp`
## Problem
The status bar title truncation loop crashes when the chapter title is
shorter than 8 characters.
```cpp
// title.length() - 8 underflows when length < 8 (size_t is unsigned)
title = title.substr(0, title.length() - 8) + "...";
```
## Fix
Added a length guard to skip truncation for titles that are too short to
truncate safely.
## Testing
- Builds successfully with `pio run`
- Affects: `src/activities/reader/EpubReaderActivity.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 😞
