fix: remove invalid RenderLock::unlock() call in end-of-book handler

RenderLock is a pure RAII wrapper with no unlock() method. The lock
releases naturally when render() returns. Build now succeeds cleanly.

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

src/activities/reader/EpubReaderActivity.cpp

@@ -844,7 +844,6 @@ void EpubReaderActivity::render(Activity::RenderLock&& lock) {
   if (currentSpineIndex == epub->getSpineItemsCount()) {
     if (!endOfBookMenuOpened) {
       endOfBookMenuOpened = true;
-      lock.unlock();
       const std::string path = epub->getPath();
       enterNewActivity(new EndOfBookMenuActivity(
           renderer, mappedInput, path, [this](EndOfBookMenuActivity::Action action) {