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

src/main.cpp

@@ -381,6 +381,12 @@ void loop() {
     screenshotButtonsReleased = true;
   }
 
+  if (activityManager.isSleepRequested()) {
+    LOG_DBG("SLP", "Activity requested sleep (push-and-sleep)");
+    enterDeepSleep();
+    return;
+  }
+
   const unsigned long sleepTimeoutMs = SETTINGS.getSleepTimeoutMs();
   if (millis() - lastActivityTime >= sleepTimeoutMs) {
     LOG_DBG("SLP", "Auto-sleep triggered after %lu ms of inactivity", sleepTimeoutMs);