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.cpp

@@ -11,7 +11,7 @@ CrossPointSettings CrossPointSettings::instance;
 namespace {
 constexpr uint8_t SETTINGS_FILE_VERSION = 1;
 // Increment this when adding new persisted settings fields
-constexpr uint8_t SETTINGS_COUNT = 5;
+constexpr uint8_t SETTINGS_COUNT = 6;
 constexpr char SETTINGS_FILE[] = "/.crosspoint/settings.bin";
 }  // namespace
 
@@ -31,6 +31,7 @@ bool CrossPointSettings::saveToFile() const {
   serialization::writePod(outputFile, shortPwrBtn);
   serialization::writePod(outputFile, statusBar);
   serialization::writePod(outputFile, orientation);
+  serialization::writePod(outputFile, frontButtonLayout);
   outputFile.close();
 
   Serial.printf("[%lu] [CPS] Settings saved to file\n", millis());
@@ -67,6 +68,8 @@ bool CrossPointSettings::loadFromFile() {
     if (++settingsRead >= fileSettingsCount) break;
     serialization::readPod(inputFile, orientation);
     if (++settingsRead >= fileSettingsCount) break;
+    serialization::readPod(inputFile, frontButtonLayout);
+    if (++settingsRead >= fileSettingsCount) break;
   } while (false);
 
   inputFile.close();