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/home/HomeActivity.h

@@ -12,6 +12,8 @@ class HomeActivity final : public Activity {
   SemaphoreHandle_t renderingMutex = nullptr;
   int selectorIndex = 0;
   bool updateRequired = false;
+  bool hasContinueReading = false;
+  const std::function<void()> onContinueReading;
   const std::function<void()> onReaderOpen;
   const std::function<void()> onSettingsOpen;
   const std::function<void()> onFileTransferOpen;
@@ -19,11 +21,14 @@ class HomeActivity final : public Activity {
   static void taskTrampoline(void* param);
   [[noreturn]] void displayTaskLoop();
   void render() const;
+  int getMenuItemCount() const;
 
  public:
-  explicit HomeActivity(GfxRenderer& renderer, InputManager& inputManager, const std::function<void()>& onReaderOpen,
+  explicit HomeActivity(GfxRenderer& renderer, InputManager& inputManager,
+                        const std::function<void()>& onContinueReading, const std::function<void()>& onReaderOpen,
                         const std::function<void()>& onSettingsOpen, const std::function<void()>& onFileTransferOpen)
       : Activity("Home", renderer, inputManager),
+        onContinueReading(onContinueReading),
         onReaderOpen(onReaderOpen),
         onSettingsOpen(onSettingsOpen),
         onFileTransferOpen(onFileTransferOpen) {}