Add navigation hints to ChapterSelectionActivities (#294)

## Summary

Add navigation hints to Chapter Select - #190 

### Before

![Mi 11X_20260108_214114_lmc_8
4](https://github.com/user-attachments/assets/45031d21-2c6c-4b7d-a5cc-6ad111bf5a70)

### After
![Mi 11X_20260108_213803_lmc_8
4](https://github.com/user-attachments/assets/1fa4ef22-63e4-4adb-8fc5-5fb8c7fa79fa)

src/activities/reader/EpubReaderChapterSelectionActivity.cpp

@@ -16,7 +16,9 @@ int EpubReaderChapterSelectionActivity::getPageItems() const {
   constexpr int lineHeight = 30;
 
   const int screenHeight = renderer.getScreenHeight();
-  const int availableHeight = screenHeight - startY;
+  const int endY = screenHeight - lineHeight;
+
+  const int availableHeight = endY - startY;
   int items = availableHeight / lineHeight;
 
   // Ensure we always have at least one item per page to avoid division by zero
@@ -134,5 +136,8 @@ void EpubReaderChapterSelectionActivity::renderScreen() {
                       tocIndex != selectorIndex);
   }
 
+  const auto labels = mappedInput.mapLabels("« Back", "Select", "Up", "Down");
+  renderer.drawButtonHints(UI_10_FONT_ID, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+
   renderer.displayBuffer();
 }