feat: Take screenshots (#759)

## Summary

* **What is the goal of this PR?** Implements a take-screenshot feature
* **What changes are included?**

- Quick press Power button and Down button at the same time to take a
screenshot
- Screenshots are saved in `screenshots` folder

## Additional Context

- Currently it does not use the device orientation.

---

Example screenshots:


![screenshot-6771.bmp](https://github.com/user-attachments/files/25157071/screenshot-6771.bmp)

[screenshot-6771.bmp](https://github.com/user-attachments/files/25157071/screenshot-6771.bmp)


![screenshot-14158.bmp](https://github.com/user-attachments/files/25157073/screenshot-14158.bmp)

[screenshot-14158.bmp](https://github.com/user-attachments/files/25157073/screenshot-14158.bmp)


### AI Usage

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

---------

Co-authored-by: Eliz Kilic <elizk@google.com>
Co-authored-by: Xuan Son Nguyen <son@huggingface.co>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Arthur Tazhitdinov <lisnake@gmail.com>

src/activities/reader/EpubReaderActivity.cpp

@@ -17,6 +17,7 @@
 #include "RecentBooksStore.h"
 #include "components/UITheme.h"
 #include "fontIds.h"
+#include "util/ScreenshotUtil.h"
 
 namespace {
 // pagesPerRefresh now comes from SETTINGS.getRefreshFrequency()
@@ -431,6 +432,15 @@ void EpubReaderActivity::onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction
       pendingGoHome = true;
       break;
     }
+    case EpubReaderMenuActivity::MenuAction::SCREENSHOT: {
+      {
+        RenderLock lock(*this);
+        pendingScreenshot = true;
+      }
+      exitActivity();
+      requestUpdate();
+      break;
+    }
     case EpubReaderMenuActivity::MenuAction::SYNC: {
       if (KOREADER_STORE.hasCredentials()) {
         const int currentPage = section ? section->currentPage : 0;
@@ -616,6 +626,11 @@ void EpubReaderActivity::render(Activity::RenderLock&& lock) {
     renderer.clearFontCache();
   }
   saveProgress(currentSpineIndex, section->currentPage, section->pageCount);
+
+  if (pendingScreenshot) {
+    pendingScreenshot = false;
+    ScreenshotUtil::takeScreenshot(renderer);
+  }
 }
 
 void EpubReaderActivity::saveProgress(int spineIndex, int currentPage, int pageCount) {