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/reader/EpubReaderChapterSelectionActivity.cpp

@@ -66,17 +66,17 @@ void EpubReaderChapterSelectionActivity::onExit() {
 }
 
 void EpubReaderChapterSelectionActivity::loop() {
-  const bool prevReleased =
-      inputManager.wasReleased(InputManager::BTN_UP) || inputManager.wasReleased(InputManager::BTN_LEFT);
-  const bool nextReleased =
-      inputManager.wasReleased(InputManager::BTN_DOWN) || inputManager.wasReleased(InputManager::BTN_RIGHT);
+  const bool prevReleased = mappedInput.wasReleased(MappedInputManager::Button::Up) ||
+                            mappedInput.wasReleased(MappedInputManager::Button::Left);
+  const bool nextReleased = mappedInput.wasReleased(MappedInputManager::Button::Down) ||
+                            mappedInput.wasReleased(MappedInputManager::Button::Right);
 
-  const bool skipPage = inputManager.getHeldTime() > SKIP_PAGE_MS;
+  const bool skipPage = mappedInput.getHeldTime() > SKIP_PAGE_MS;
   const int pageItems = getPageItems();
 
-  if (inputManager.wasReleased(InputManager::BTN_CONFIRM)) {
+  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
     onSelectSpineIndex(selectorIndex);
-  } else if (inputManager.wasReleased(InputManager::BTN_BACK)) {
+  } else if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
     onGoBack();
   } else if (prevReleased) {
     if (skipPage) {