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/ReaderActivity.h

@@ -21,9 +21,9 @@ class ReaderActivity final : public ActivityWithSubactivity {
   void onGoToXtcReader(std::unique_ptr<Xtc> xtc);
 
  public:
-  explicit ReaderActivity(GfxRenderer& renderer, InputManager& inputManager, std::string initialBookPath,
+  explicit ReaderActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, std::string initialBookPath,
                           const std::function<void()>& onGoBack)
-      : ActivityWithSubactivity("Reader", renderer, inputManager),
+      : ActivityWithSubactivity("Reader", renderer, mappedInput),
         initialBookPath(std::move(initialBookPath)),
         onGoBack(onGoBack) {}
   void onEnter() override;