refactor: rename MyLibrary to FileBrowser (#1260)

## Summary

* Renames MyLibrary component to FileBrowser, as it better reflects what
it is, in my opinion

## Additional Context

* Frees the Library name for possible future library component that can
cache metadata, provide other ways of browsing than filesystem
structure, etc
---

### AI Usage

Did you use AI tools to help write this code? _**< YES >**_

src/activities/reader/EpubReaderActivity.cpp

@@ -180,7 +180,7 @@ void EpubReaderActivity::loop() {
 
   // Long press BACK (1s+) goes to file selection
   if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= goHomeMs) {
-    activityManager.goToMyLibrary(epub ? epub->getPath() : "");
+    activityManager.goToFileBrowser(epub ? epub->getPath() : "");
     return;
   }