feat: unify navigation handling with system-wide continuous navigation (#600)

This PR unifies navigation handling & adds system-wide support for
continuous navigation.

## Summary
Holding down a navigation button now continuously advances through items
until the button is released. This removes the need for repeated
press-and-release actions and makes navigation faster and smoother,
especially in long menus or documents.

When page-based navigation is available, it will navigate through pages.
If not, it will progress through menu items or similar list-based UI
elements.

Additionally, this PR fixes inconsistencies in wrap-around behavior and
navigation index calculations.

Places where the navigation system was updated:
- Home Page
- Settings Pages
- My Library Page
- WiFi Selection Page
- OPDS Browser Page
- Keyboard
- File Transfer Page
- XTC Chapter Selector Page
- EPUB Chapter Selector Page

I’ve tested this on the device as much as possible and tried to match
the existing behavior. Please let me know if I missed anything. Thanks 🙏


![crosspoint](https://github.com/user-attachments/assets/6a3c7482-f45e-4a77-b156-721bb3b679e6)

---

Following the request from @osteotek and @daveallie for system-wide
support, the old PR (#379) has been closed in favor of this
consolidated, system-wide implementation.

---

### AI Usage

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

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/activities/reader/EpubReaderMenuActivity.cpp

@@ -48,16 +48,19 @@ void EpubReaderMenuActivity::loop() {
     return;
   }
 
+  // Handle navigation
+  buttonNavigator.onNext([this] {
+    selectedIndex = ButtonNavigator::nextIndex(selectedIndex, static_cast<int>(menuItems.size()));
+    updateRequired = true;
+  });
+
+  buttonNavigator.onPrevious([this] {
+    selectedIndex = ButtonNavigator::previousIndex(selectedIndex, static_cast<int>(menuItems.size()));
+    updateRequired = true;
+  });
+
   // Use local variables for items we need to check after potential deletion
-  if (mappedInput.wasReleased(MappedInputManager::Button::Up) ||
-      mappedInput.wasReleased(MappedInputManager::Button::Left)) {
-    selectedIndex = (selectedIndex + menuItems.size() - 1) % menuItems.size();
-    updateRequired = true;
-  } else if (mappedInput.wasReleased(MappedInputManager::Button::Down) ||
-             mappedInput.wasReleased(MappedInputManager::Button::Right)) {
-    selectedIndex = (selectedIndex + 1) % menuItems.size();
-    updateRequired = true;
-  } else if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
     const auto selectedAction = menuItems[selectedIndex].action;
     if (selectedAction == MenuAction::ROTATE_SCREEN) {
       // Cycle orientation preview locally; actual rotation happens on menu exit.