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/ReaderActivity.h

@@ -7,11 +7,13 @@ class Epub;
 
 class ReaderActivity final : public ActivityWithSubactivity {
   std::string initialEpubPath;
+  std::string currentEpubPath;  // Track current book path for navigation
   const std::function<void()> onGoBack;
   static std::unique_ptr<Epub> loadEpub(const std::string& path);
 
+  static std::string extractFolderPath(const std::string& filePath);
   void onSelectEpubFile(const std::string& path);
-  void onGoToFileSelection();
+  void onGoToFileSelection(const std::string& fromEpubPath = "");
   void onGoToEpubReader(std::unique_ptr<Epub> epub);
 
  public: