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/CrossPointSettings.h

@@ -28,6 +28,11 @@ class CrossPointSettings {
     LANDSCAPE_CCW = 3  // 800x480 logical coordinates, native panel orientation
   };
 
+  // Front button layout options
+  // Default: Back, Confirm, Left, Right
+  // Swapped: Left, Right, Back, Confirm
+  enum FRONT_BUTTON_LAYOUT { BACK_CONFIRM_LEFT_RIGHT = 0, LEFT_RIGHT_BACK_CONFIRM = 1 };
+
   // Sleep screen settings
   uint8_t sleepScreen = DARK;
   // Status bar settings
@@ -39,6 +44,8 @@ class CrossPointSettings {
   // EPUB reading orientation settings
   // 0 = portrait (default), 1 = landscape clockwise, 2 = inverted, 3 = landscape counter-clockwise
   uint8_t orientation = PORTRAIT;
+  // Front button layout
+  uint8_t frontButtonLayout = BACK_CONFIRM_LEFT_RIGHT;
 
   ~CrossPointSettings() = default;