## Summary
* **What is the goal of this PR?**
Add a "Continue Reading" feature to improve user experience when
returning to a previously opened book.
* **What changes are included?**
- Add dynamic "Continue: <book name>" menu item in Home screen when a
book was previously opened
- File browser now starts from the folder of the last opened book
instead of always starting from root directory
- Menu dynamically shows 3 or 4 items based on reading history:
- Without history: `Browse`, `File transfer`, `Settings`
- With history: `Continue: <book>`, `Browse`, `File transfer`,
`Settings`
## Additional Context
* This feature leverages the existing `APP_STATE.openEpubPath` which
already persists the last opened book path
* The Continue Reading menu only appears if the book file still exists
on the SD card
* Book name in the menu is truncated to 25 characters with "..." suffix
if too long
* If the last book's folder was deleted, the file browser gracefully
falls back to root directory
* No new dependencies or significant memory overhead - reuses existing
state management
## Summary
* Give activities name and log when entering and exiting them
* Clearer logs when attempting to debug, knowing where users are coming
from/going to helps
## Summary
* Unset openEpubPath on boot and set once epub fully loaded
## Additional Context
* If an epub was crashing when loading, it was possible to get the
device stuck into a loop. There was no way to get back to the home
screen as we'd always load you back into old epub
* Break this loop by clearing the stored value when we boot, still
jumping to the last open epub, but only resetting that value once the
epub has been fully loaded
## 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 😞
