feat: add EndOfBookMenuActivity replacing static end-of-book text

Interactive menu shown when reaching the end of a book with options:
Archive Book, Delete Book, Back to Beginning, Close Book, Close Menu.
Wired into EpubReaderActivity, XtcReaderActivity, and TxtReaderActivity
(TXT shows menu when user tries to advance past the last page).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/activities/reader/XtcReaderActivity.h

@@ -17,6 +17,7 @@ class XtcReaderActivity final : public ActivityWithSubactivity {
   uint32_t currentPage = 0;
   int pagesUntilFullRefresh = 0;
 
+  bool endOfBookMenuOpened = false;
   const std::function<void()> onGoBack;
   const std::function<void()> onGoHome;