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/FileSelectionActivity.h

@@ -25,10 +25,10 @@ class FileSelectionActivity final : public Activity {
   void loadFiles();
 
  public:
-  explicit FileSelectionActivity(GfxRenderer& renderer, InputManager& inputManager,
+  explicit FileSelectionActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
                                  const std::function<void(const std::string&)>& onSelect,
                                  const std::function<void()>& onGoHome, std::string initialPath = "/")
-      : Activity("FileSelection", renderer, inputManager),
+      : Activity("FileSelection", renderer, mappedInput),
         basepath(initialPath.empty() ? "/" : std::move(initialPath)),
         onSelect(onSelect),
         onGoHome(onGoHome) {}