Support swapping the functionality of the front buttons (#133)

## Summary

**What is the goal of this PR?** 

Adds a setting to swap the front buttons. The default functionality are:
Back/Confirm/Left/Right. When this setting is enabled they become:
Left/Right/Back/Confirm. This makes it more comfortable to use when
holding in your right hand since your thumb can more easily rest on the
next button. The original firmware has a similar setting.

**What changes are included?**

- Add the new setting.
- Create a mapper to dynamically switch the buttons based on the
setting.
- Use mapper on the various activity screens.
- Update the button hints to reflect the swapped buttons.

## Additional Context

Full disclosure: I used Codex CLI to put this PR together, but did
review it to make sure it makes sense.

Also tested on my device:
https://share.cleanshot.com/k76891NY

src/activities/home/HomeActivity.cpp

@@ -49,14 +49,14 @@ void HomeActivity::onExit() {
 }
 
 void HomeActivity::loop() {
-  const bool prevPressed =
-      inputManager.wasPressed(InputManager::BTN_UP) || inputManager.wasPressed(InputManager::BTN_LEFT);
-  const bool nextPressed =
-      inputManager.wasPressed(InputManager::BTN_DOWN) || inputManager.wasPressed(InputManager::BTN_RIGHT);
+  const bool prevPressed = mappedInput.wasPressed(MappedInputManager::Button::Up) ||
+                           mappedInput.wasPressed(MappedInputManager::Button::Left);
+  const bool nextPressed = mappedInput.wasPressed(MappedInputManager::Button::Down) ||
+                           mappedInput.wasPressed(MappedInputManager::Button::Right);
 
   const int menuCount = getMenuItemCount();
 
-  if (inputManager.wasReleased(InputManager::BTN_CONFIRM)) {
+  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
     if (hasContinueReading) {
       // Menu: Continue Reading, Browse, File transfer, Settings
       if (selectorIndex == 0) {
@@ -143,7 +143,8 @@ void HomeActivity::render() const {
 
   renderer.drawText(UI_FONT_ID, 20, menuY, "Settings", selectorIndex != menuIndex);
 
-  renderer.drawButtonHints(UI_FONT_ID, "Back", "Confirm", "Left", "Right");
+  const auto labels = mappedInput.mapLabels("Back", "Confirm", "Left", "Right");
+  renderer.drawButtonHints(UI_FONT_ID, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
 
   renderer.displayBuffer();
 }