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.
