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/MappedInputManager.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <InputManager.h>
+
+#include "CrossPointSettings.h"
+
+class MappedInputManager {
+ public:
+  enum class Button { Back, Confirm, Left, Right, Up, Down, Power, PageBack, PageForward };
+
+  struct Labels {
+    const char* btn1;
+    const char* btn2;
+    const char* btn3;
+    const char* btn4;
+  };
+
+  explicit MappedInputManager(InputManager& inputManager) : inputManager(inputManager) {}
+
+  bool wasPressed(Button button) const;
+  bool wasReleased(Button button) const;
+  bool isPressed(Button button) const;
+  bool wasAnyPressed() const;
+  bool wasAnyReleased() const;
+  unsigned long getHeldTime() const;
+  Labels mapLabels(const char* back, const char* confirm, const char* previous, const char* next) const;
+
+ private:
+  InputManager& inputManager;
+  decltype(InputManager::BTN_BACK) mapButton(Button button) const;
+};