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/activities/reader/TxtReaderActivity.cpp

@@ -60,16 +60,8 @@ void TxtReaderActivity::onExit() {
 }
 
 void TxtReaderActivity::loop() {
-  // Long press BACK (1s+) goes to file selection
-  if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= ReaderUtils::GO_HOME_MS) {
-    activityManager.goToFileBrowser(txt ? txt->getPath() : "");
-    return;
-  }
-
-  // Short press BACK goes directly to home
-  if (mappedInput.wasReleased(MappedInputManager::Button::Back) &&
-      mappedInput.getHeldTime() < ReaderUtils::GO_HOME_MS) {
-    onGoHome();
+  if (ReaderUtils::handleBackNavigation(mappedInput, activityManager, txt ? txt->getPath().c_str() : "",
+                                        {this, [](void* ctx) { static_cast<TxtReaderActivity*>(ctx)->onGoHome(); }})) {
     return;
   }