feat: Add option to switch behavior for "back to browser / home" in Reader activity (#2366)

## Summary

It would be nice to switch back to the file list from an Reader activity
via a short back button press. This change adds an Reader option to
switch the default behavior, so a short back button press in the Reader
activity can now go back to the file list, and a long press on back goes
back to the home view. This does a fair bit of refactoring, introducing
a new constant for the ms limit.

* **What changes are included?**

- Changes to the translation
- Additional global Reader option 
- Refactoring of the back button behavior in the Reader activity

src/CrossPointSettings.h

@@ -271,6 +271,8 @@ class CrossPointSettings {
   uint8_t removeReadBooksFromRecents = 0;
   // Move epub to /Read/ folder on SD card when finished (0 = disabled, 1 = enabled)
   uint8_t moveFinishedToReadFolder = 0;
+  // Short press Back goes to file browser instead of home (0 = disabled, 1 = enabled)
+  uint8_t backShortToFileBrowser = 0;
   // Image rendering mode in EPUB reader
   uint8_t imageRendering = IMAGES_DISPLAY;
   // Tilt-based page turning (X3 only — requires QMI8658 IMU)