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/FileBrowserActivity.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+#include "RecentBooksStore.h"
+#include "util/ButtonNavigator.h"
+
+class FileBrowserActivity final : public Activity {
+ private:
+  // Deletion
+  void clearFileMetadata(const std::string& fullPath);
+
+  ButtonNavigator buttonNavigator;
+
+  size_t selectorIndex = 0;
+
+  // Files state
+  std::string basepath = "/";
+  std::vector<std::string> files;
+
+  // Data loading
+  void loadFiles();
+  size_t findEntry(const std::string& name) const;
+
+ public:
+  explicit FileBrowserActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, std::string initialPath = "/")
+      : Activity("FileBrowser", renderer, mappedInput), basepath(initialPath.empty() ? "/" : std::move(initialPath)) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+  void render(RenderLock&&) override;
+};