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/reader/ReaderActivity.cpp

@@ -80,7 +80,7 @@ std::unique_ptr<Txt> ReaderActivity::loadTxt(const std::string& path) {
 void ReaderActivity::goToLibrary(const std::string& fromBookPath) {
   // If coming from a book, start in that book's folder; otherwise start from root
   auto initialPath = fromBookPath.empty() ? "/" : extractFolderPath(fromBookPath);
-  activityManager.goToMyLibrary(std::move(initialPath));
+  activityManager.goToFileBrowser(std::move(initialPath));
 }
 
 void ReaderActivity::onGoToEpubReader(std::unique_ptr<Epub> epub) {