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/home/HomeActivity.h

@@ -3,7 +3,7 @@
 #include <vector>
 
 #include "../Activity.h"
-#include "./MyLibraryActivity.h"
+#include "./FileBrowserActivity.h"
 #include "util/ButtonNavigator.h"
 
 struct RecentBook;
@@ -21,7 +21,7 @@ class HomeActivity final : public Activity {
   uint8_t* coverBuffer = nullptr;  // HomeActivity's own buffer for cover image
   std::vector<RecentBook> recentBooks;
   void onSelectBook(const std::string& path);
-  void onMyLibraryOpen();
+  void onFileBrowserOpen();
   void onRecentsOpen();
   void onSettingsOpen();
   void onFileTransferOpen();