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/XtcReaderActivity.h

@@ -32,9 +32,9 @@ class XtcReaderActivity final : public Activity {
   void loadProgress();
 
  public:
-  explicit XtcReaderActivity(GfxRenderer& renderer, InputManager& inputManager, std::unique_ptr<Xtc> xtc,
+  explicit XtcReaderActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, std::unique_ptr<Xtc> xtc,
                              const std::function<void()>& onGoBack, const std::function<void()>& onGoHome)
-      : Activity("XtcReader", renderer, inputManager), xtc(std::move(xtc)), onGoBack(onGoBack), onGoHome(onGoHome) {}
+      : Activity("XtcReader", renderer, mappedInput), xtc(std::move(xtc)), onGoBack(onGoBack), onGoHome(onGoHome) {}
   void onEnter() override;
   void onExit() override;
   void loop() override;