feat: Add per-book letterbox fill override

Introduce BookSettings utility for per-book settings stored in
the book's cache directory (book_settings.bin). Add "Letterbox Fill"
option to the EPUB reader menu that cycles Default/Dithered/Solid/None.
At sleep time, the per-book override is loaded and takes precedence
over the global setting for all book types (EPUB, XTC, TXT).

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

src/activities/reader/EpubReaderActivity.cpp

@@ -242,7 +242,7 @@ void EpubReaderActivity::loop() {
     exitActivity();
     enterNewActivity(new EpubReaderMenuActivity(
         this->renderer, this->mappedInput, epub->getTitle(), currentPage, totalPages, bookProgressPercent,
-        SETTINGS.orientation, hasDictionary, isBookmarked,
+        SETTINGS.orientation, hasDictionary, isBookmarked, epub->getCachePath(),
         [this](const uint8_t orientation) { onReaderMenuBack(orientation); },
         [this](EpubReaderMenuActivity::MenuAction action) { onReaderMenuConfirm(action); }));
     xSemaphoreGive(renderingMutex);
@@ -776,6 +776,10 @@ void EpubReaderActivity::onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction
       }
       break;
     }
+    // Handled locally in the menu activity (cycle on Confirm, never dispatched here)
+    case EpubReaderMenuActivity::MenuAction::ROTATE_SCREEN:
+    case EpubReaderMenuActivity::MenuAction::LETTERBOX_FILL:
+      break;
   }
 }