Add Continue Reading menu and remember last book folder (#129)

## Summary

* **What is the goal of this PR?**

Add a "Continue Reading" feature to improve user experience when
returning to a previously opened book.

* **What changes are included?**

- Add dynamic "Continue: <book name>" menu item in Home screen when a
book was previously opened

- File browser now starts from the folder of the last opened book
instead of always starting from root directory
- Menu dynamically shows 3 or 4 items based on reading history:
  - Without history: `Browse`, `File transfer`, `Settings`
- With history: `Continue: <book>`, `Browse`, `File transfer`,
`Settings`

## Additional Context

* This feature leverages the existing `APP_STATE.openEpubPath` which
already persists the last opened book path
* The Continue Reading menu only appears if the book file still exists
on the SD card
* Book name in the menu is truncated to 25 characters with "..." suffix
if too long
* If the last book's folder was deleted, the file browser gracefully
falls back to root directory
* No new dependencies or significant memory overhead - reuses existing
state management

src/activities/reader/EpubReaderActivity.h

@@ -17,6 +17,7 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   int pagesUntilFullRefresh = 0;
   bool updateRequired = false;
   const std::function<void()> onGoBack;
+  const std::function<void()> onGoHome;
 
   static void taskTrampoline(void* param);
   [[noreturn]] void displayTaskLoop();
@@ -26,8 +27,11 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
 
  public:
   explicit EpubReaderActivity(GfxRenderer& renderer, InputManager& inputManager, std::unique_ptr<Epub> epub,
-                              const std::function<void()>& onGoBack)
-      : ActivityWithSubactivity("EpubReader", renderer, inputManager), epub(std::move(epub)), onGoBack(onGoBack) {}
+                              const std::function<void()>& onGoBack, const std::function<void()>& onGoHome)
+      : ActivityWithSubactivity("EpubReader", renderer, inputManager),
+        epub(std::move(epub)),
+        onGoBack(onGoBack),
+        onGoHome(onGoHome) {}
   void onEnter() override;
   void onExit() override;
   void loop() override;