mod: restore missing mod features from resync audit

Re-add KOReaderSyncActivity PUSH_ONLY mode (PR #1090):
- SyncMode enum with INTERACTIVE/PUSH_ONLY, deferFinish pattern
- Push & Sleep menu action in EpubReaderMenuActivity
- ActivityManager::requestSleep() for activity-initiated sleep
- main.cpp checks isSleepRequested() each loop iteration

Wire EndOfBookMenuActivity into EpubReaderActivity:
- pendingEndOfBookMenu deferred flag avoids render-lock deadlock
- Handles all 6 actions: ARCHIVE, DELETE, TABLE_OF_CONTENTS,
  BACK_TO_BEGINNING, CLOSE_BOOK, CLOSE_MENU

Add book management to reader menu:
- ARCHIVE_BOOK, DELETE_BOOK, REINDEX_BOOK actions with handlers

Port silent next-chapter pre-indexing:
- silentIndexNextChapterIfNeeded() proactively indexes next chapter
  when user is near end of current one, eliminating load screens

Add per-book letterbox fill toggle in reader menu:
- LETTERBOX_FILL cycles Default/Dithered/Solid/None
- Loads/saves per-book override via BookSettings
- bookCachePath constructor param added to EpubReaderMenuActivity

Made-with: Cursor

lib/I18n/translations/english.yaml

@@ -300,6 +300,7 @@ STR_HW_RIGHT_LABEL: "Right (4th button)"
 STR_GO_TO_PERCENT: "Go to %"
 STR_GO_HOME_BUTTON: "Go Home"
 STR_SYNC_PROGRESS: "Sync Progress"
+STR_PUSH_AND_SLEEP: "Push & Sleep"
 STR_DELETE_CACHE: "Delete Book Cache"
 STR_DELETE: "Delete"
 STR_DISPLAY_QR: "Show page as QR"