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.h

@@ -2,7 +2,7 @@
 #include <memory>
 
 #include "../Activity.h"
-#include "activities/home/MyLibraryActivity.h"
+#include "activities/home/FileBrowserActivity.h"
 
 class Epub;
 class Xtc;