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/Activity.h

@@ -5,18 +5,19 @@
 #include <string>
 #include <utility>
 
-class InputManager;
+#include "../MappedInputManager.h"
+
 class GfxRenderer;
 
 class Activity {
  protected:
   std::string name;
   GfxRenderer& renderer;
-  InputManager& inputManager;
+  MappedInputManager& mappedInput;
 
  public:
-  explicit Activity(std::string name, GfxRenderer& renderer, InputManager& inputManager)
-      : name(std::move(name)), renderer(renderer), inputManager(inputManager) {}
+  explicit Activity(std::string name, GfxRenderer& renderer, MappedInputManager& mappedInput)
+      : name(std::move(name)), renderer(renderer), mappedInput(mappedInput) {}
   virtual ~Activity() = default;
   virtual void onEnter() { Serial.printf("[%lu] [ACT] Entering activity: %s\n", millis(), name.c_str()); }
   virtual void onExit() { Serial.printf("[%lu] [ACT] Exiting activity: %s\n", millis(), name.c_str()); }