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/network/WifiSelectionActivity.cpp

@@ -420,20 +420,16 @@ void WifiSelectionActivity::loop() {
       return;
     }
 
-    // Handle UP/DOWN navigation
-    if (mappedInput.wasPressed(MappedInputManager::Button::Up) ||
-        mappedInput.wasPressed(MappedInputManager::Button::Left)) {
-      if (selectedNetworkIndex > 0) {
-        selectedNetworkIndex--;
-        updateRequired = true;
-      }
-    } else if (mappedInput.wasPressed(MappedInputManager::Button::Down) ||
-               mappedInput.wasPressed(MappedInputManager::Button::Right)) {
-      if (!networks.empty() && selectedNetworkIndex < static_cast<int>(networks.size()) - 1) {
-        selectedNetworkIndex++;
-        updateRequired = true;
-      }
-    }
+    // Handle navigation
+    buttonNavigator.onNext([this] {
+      selectedNetworkIndex = ButtonNavigator::nextIndex(selectedNetworkIndex, networks.size());
+      updateRequired = true;
+    });
+
+    buttonNavigator.onPrevious([this] {
+      selectedNetworkIndex = ButtonNavigator::previousIndex(selectedNetworkIndex, networks.size());
+      updateRequired = true;
+    });
   }
 }