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/util/KeyboardEntryActivity.cpp

@@ -138,7 +138,7 @@ void KeyboardEntryActivity::handleKeyPress() {
 
 void KeyboardEntryActivity::loop() {
   // Navigation
-  if (inputManager.wasPressed(InputManager::BTN_UP)) {
+  if (mappedInput.wasPressed(MappedInputManager::Button::Up)) {
     if (selectedRow > 0) {
       selectedRow--;
       // Clamp column to valid range for new row
@@ -148,7 +148,7 @@ void KeyboardEntryActivity::loop() {
     updateRequired = true;
   }
 
-  if (inputManager.wasPressed(InputManager::BTN_DOWN)) {
+  if (mappedInput.wasPressed(MappedInputManager::Button::Down)) {
     if (selectedRow < NUM_ROWS - 1) {
       selectedRow++;
       const int maxCol = getRowLength(selectedRow) - 1;
@@ -157,7 +157,7 @@ void KeyboardEntryActivity::loop() {
     updateRequired = true;
   }
 
-  if (inputManager.wasPressed(InputManager::BTN_LEFT)) {
+  if (mappedInput.wasPressed(MappedInputManager::Button::Left)) {
     // Special bottom row case
     if (selectedRow == SPECIAL_ROW) {
       // Bottom row has special key widths
@@ -187,7 +187,7 @@ void KeyboardEntryActivity::loop() {
     updateRequired = true;
   }
 
-  if (inputManager.wasPressed(InputManager::BTN_RIGHT)) {
+  if (mappedInput.wasPressed(MappedInputManager::Button::Right)) {
     const int maxCol = getRowLength(selectedRow) - 1;
 
     // Special bottom row case
@@ -220,13 +220,13 @@ void KeyboardEntryActivity::loop() {
   }
 
   // Selection
-  if (inputManager.wasPressed(InputManager::BTN_CONFIRM)) {
+  if (mappedInput.wasPressed(MappedInputManager::Button::Confirm)) {
     handleKeyPress();
     updateRequired = true;
   }
 
   // Cancel
-  if (inputManager.wasPressed(InputManager::BTN_BACK)) {
+  if (mappedInput.wasPressed(MappedInputManager::Button::Back)) {
     if (onCancel) {
       onCancel();
     }