fix: Show "Back" in file browser if not in root, "Home" otherwise. (#822)

## Summary

Show "Back" in file browser if not in root, "Home" otherwise.

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### AI Usage

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Did you use AI tools to help write this code? YES

src/activities/home/MyLibraryActivity.cpp

@@ -246,7 +246,7 @@ void MyLibraryActivity::render() const {
   }
 
   // Help text
-  const auto labels = mappedInput.mapLabels("« Home", "Open", "Up", "Down");
+  const auto labels = mappedInput.mapLabels(basepath == "/" ? "« Home" : "« Back", "Open", "Up", "Down");
   GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
 
   renderer.displayBuffer();