Add support for XTC (XTeink X4 native) ebook format with pre-rendered
bitmap pages. Key changes:
- Add Xtc library with XtcParser for reading XTC files
- XTC format: 22-byte XTG page headers with 480x800 1-bit bitmaps
- XtcReaderActivity for displaying XTC pages on e-ink display
- Correct bit polarity: 0=black, 1=white in XTC format
- FileSelectionActivity: detect and handle .xtc files
- ReaderActivity: route to XtcReaderActivity for XTC files
- Cover BMP generation from first XTC page
XTC pages include pre-rendered status bar with page numbers and
progress, so no additional overlay is needed.
## 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
* 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 😞
