feat: add EndOfBookMenuActivity replacing static end-of-book text

Interactive menu shown when reaching the end of a book with options:
Archive Book, Delete Book, Back to Beginning, Close Book, Close Menu.
Wired into EpubReaderActivity, XtcReaderActivity, and TxtReaderActivity
(TXT shows menu when user tries to advance past the last page).

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

src/activities/reader/EndOfBookMenuActivity.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <I18n.h>
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+#include "util/ButtonNavigator.h"
+
+class EndOfBookMenuActivity final : public Activity {
+ public:
+  enum class Action {
+    ARCHIVE,
+    DELETE,
+    BACK_TO_BEGINNING,
+    CLOSE_BOOK,
+    CLOSE_MENU,
+  };
+
+  explicit EndOfBookMenuActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, const std::string& bookPath,
+                                 const std::function<void(Action)>& onAction)
+      : Activity("EndOfBookMenu", renderer, mappedInput), bookPath(bookPath), onAction(onAction) {
+    buildMenuItems();
+  }
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+  void render(Activity::RenderLock&&) override;
+
+ private:
+  struct MenuItem {
+    Action action;
+    StrId labelId;
+  };
+
+  std::string bookPath;
+  std::vector<MenuItem> menuItems;
+  int selectedIndex = 0;
+  ButtonNavigator buttonNavigator;
+  const std::function<void(Action)> onAction;
+
+  void buildMenuItems();
+};