feat: Move Sync feature to menu (#680)

## Summary

* **What is the goal of this PR?** 
Move the "Sync Progress" option from TOC (Chapter Selection) screen to
the Reader Menu, and fix use-after-free crashes related to callback
handling in activity lifecycle.

* **What changes are included?**
- Added "Sync Progress" as a menu item in `EpubReaderMenuActivity` (now
4 items: Go to Chapter, Sync Progress, Go Home, Delete Book Cache)
- Removed sync-related logic from `EpubReaderChapterSelectionActivity` -
TOC now only displays chapters
- Implemented `pendingGoHome` and `pendingSubactivityExit` flags in
`EpubReaderActivity` to safely handle activity destruction
- Fixed GO_HOME, DELETE_CACHE, and SYNC menu actions to use deferred
callbacks avoiding use-after-free

## Additional Context

* Root cause of crashes: callbacks like `onGoHome()` or `onCancel()`
invoked from activity handlers could destroy the current activity while
code was still executing, causing use-after-free and race conditions
with FreeRTOS display task.
* Solution: Deferred execution pattern - set flags and process them in
`loop()` after all nested activity loops have safely returned.
* Files changed: `EpubReaderMenuActivity.h`,
`EpubReaderActivity.h/.cpp`, `EpubReaderChapterSelectionActivity.h/.cpp`

---

### AI Usage

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please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**YES**_

Co-authored-by: danoooob <danoooob@example.com>
Co-authored-by: Dave Allie <dave@daveallie.com>

src/activities/reader/EpubReaderChapterSelectionActivity.h

@@ -26,22 +26,12 @@ class EpubReaderChapterSelectionActivity final : public ActivityWithSubactivity
   // This adapts automatically when switching between portrait and landscape.
   int getPageItems() const;
 
-  // Total items including sync options (top and bottom)
+  // Total TOC items count
   int getTotalItems() const;
 
-  // Check if sync option is available (credentials configured)
-  bool hasSyncOption() const;
-
-  // Check if given item index is a sync option (first or last)
-  bool isSyncItem(int index) const;
-
-  // Convert item index to TOC index (accounting for top sync option offset)
-  int tocIndexFromItemIndex(int itemIndex) const;
-
   static void taskTrampoline(void* param);
   [[noreturn]] void displayTaskLoop();
   void renderScreen();
-  void launchSyncActivity();
 
  public:
   explicit EpubReaderChapterSelectionActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,