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/reader/EpubReaderActivity.cpp

@@ -180,7 +180,7 @@ void EpubReaderActivity::loop() {
 
   // Long press BACK (1s+) goes to file selection
   if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= goHomeMs) {
-    activityManager.goToMyLibrary(epub ? epub->getPath() : "");
+    activityManager.goToFileBrowser(epub ? epub->getPath() : "");
     return;
   }
 

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) {

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;

src/activities/reader/TxtReaderActivity.cpp

@@ -76,7 +76,7 @@ void TxtReaderActivity::onExit() {
 void TxtReaderActivity::loop() {
   // Long press BACK (1s+) goes to file selection
   if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= goHomeMs) {
-    activityManager.goToMyLibrary(txt ? txt->getPath() : "");
+    activityManager.goToFileBrowser(txt ? txt->getPath() : "");
     return;
   }
 

src/activities/reader/XtcReaderActivity.cpp

@@ -70,7 +70,7 @@ void XtcReaderActivity::loop() {
 
   // Long press BACK (1s+) goes to file selection
   if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= goHomeMs) {
-    activityManager.goToMyLibrary(xtc ? xtc->getPath() : "");
+    activityManager.goToFileBrowser(xtc ? xtc->getPath() : "");
     return;
   }