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.cpp

@@ -71,21 +71,21 @@ void XtcReaderActivity::onExit() {
 
 void XtcReaderActivity::loop() {
   // Long press BACK (1s+) goes directly to home
-  if (inputManager.isPressed(InputManager::BTN_BACK) && inputManager.getHeldTime() >= goHomeMs) {
+  if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= goHomeMs) {
     onGoHome();
     return;
   }
 
   // Short press BACK goes to file selection
-  if (inputManager.wasReleased(InputManager::BTN_BACK) && inputManager.getHeldTime() < goHomeMs) {
+  if (mappedInput.wasReleased(MappedInputManager::Button::Back) && mappedInput.getHeldTime() < goHomeMs) {
     onGoBack();
     return;
   }
 
-  const bool prevReleased =
-      inputManager.wasReleased(InputManager::BTN_UP) || inputManager.wasReleased(InputManager::BTN_LEFT);
-  const bool nextReleased =
-      inputManager.wasReleased(InputManager::BTN_DOWN) || inputManager.wasReleased(InputManager::BTN_RIGHT);
+  const bool prevReleased = mappedInput.wasReleased(MappedInputManager::Button::PageBack) ||
+                            mappedInput.wasReleased(MappedInputManager::Button::Left);
+  const bool nextReleased = mappedInput.wasReleased(MappedInputManager::Button::PageForward) ||
+                            mappedInput.wasReleased(MappedInputManager::Button::Right);
 
   if (!prevReleased && !nextReleased) {
     return;
@@ -98,7 +98,7 @@ void XtcReaderActivity::loop() {
     return;
   }
 
-  const bool skipPages = inputManager.getHeldTime() > skipPageMs;
+  const bool skipPages = mappedInput.getHeldTime() > skipPageMs;
   const int skipAmount = skipPages ? 10 : 1;
 
   if (prevReleased) {