refactor: change browse activities to ActivityWithSubactivity

Change HomeActivity, MyLibraryActivity, and RecentBooksActivity base
class from Activity to ActivityWithSubactivity. Adds subActivity
guard at top of each loop(). No new behavior, just enabling sub-activity
hosting for the upcoming BookManageMenuActivity integration.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/activities/home/MyLibraryActivity.cpp

@@ -103,7 +103,7 @@ void MyLibraryActivity::loadFiles() {
 }
 
 void MyLibraryActivity::onEnter() {
-  Activity::onEnter();
+  ActivityWithSubactivity::onEnter();
 
   loadFiles();
   selectorIndex = 0;
@@ -112,11 +112,16 @@ void MyLibraryActivity::onEnter() {
 }
 
 void MyLibraryActivity::onExit() {
-  Activity::onExit();
+  ActivityWithSubactivity::onExit();
   files.clear();
 }
 
 void MyLibraryActivity::loop() {
+  if (subActivity) {
+    subActivity->loop();
+    return;
+  }
+
   // Long press BACK (1s+) goes to root folder
   if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= GO_HOME_MS &&
       basepath != "/") {