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/activities/ActivityManager.h

@@ -63,6 +63,8 @@ class ActivityManager {
   // This variable must only be set by the main loop, to avoid race conditions
   bool requestedUpdate = false;
 
+  bool sleepRequested = false;
+
  public:
   explicit ActivityManager(GfxRenderer& renderer, MappedInputManager& mappedInput)
       : renderer(renderer), mappedInput(mappedInput), renderingMutex(xSemaphoreCreateMutex()) {
@@ -101,6 +103,11 @@ class ActivityManager {
   bool isReaderActivity() const;
   bool skipLoopDelay() const;
 
+  // Activities can request sleep (e.g. PUSH_AND_SLEEP). The main loop checks
+  // this flag after each loop() call and triggers enterDeepSleep() if set.
+  void requestSleep() { sleepRequested = true; }
+  bool isSleepRequested() const { return sleepRequested; }
+
   // If immediate is true, the update will be triggered immediately.
   // Otherwise, it will be deferred until the end of the current loop iteration.
   void requestUpdate(bool immediate = false);