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.h

@@ -20,7 +20,8 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   float pendingSpineProgress = 0.0f;
   bool pendingSubactivityExit = false;  // Defer subactivity exit to avoid use-after-free
   bool pendingGoHome = false;           // Defer go home to avoid race condition with display task
-  bool skipNextButtonCheck = false;     // Skip button processing for one frame after subactivity exit
+  bool pendingScreenshot = false;
+  bool skipNextButtonCheck = false;  // Skip button processing for one frame after subactivity exit
   const std::function<void()> onGoBack;
   const std::function<void()> onGoHome;