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
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### AI Usage

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

src/activities/ActivityManager.h

@@ -79,7 +79,7 @@ class ActivityManager {
   // goTo... functions are convenient wrapper for replaceActivity()
   void goToFileTransfer();
   void goToSettings();
-  void goToMyLibrary(std::string path = {});
+  void goToFileBrowser(std::string path = {});
   void goToRecentBooks();
   void goToBrowser();
   void goToReader(std::string path);