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/FileSelectionActivity.h

@@ -27,8 +27,11 @@ class FileSelectionActivity final : public Activity {
  public:
   explicit FileSelectionActivity(GfxRenderer& renderer, InputManager& inputManager,
                                  const std::function<void(const std::string&)>& onSelect,
-                                 const std::function<void()>& onGoHome)
-      : Activity("FileSelection", renderer, inputManager), onSelect(onSelect), onGoHome(onGoHome) {}
+                                 const std::function<void()>& onGoHome, std::string initialPath = "/")
+      : Activity("FileSelection", renderer, inputManager),
+        basepath(initialPath.empty() ? "/" : std::move(initialPath)),
+        onSelect(onSelect),
+        onGoHome(onGoHome) {}
   void onEnter() override;
   void onExit() override;
   void loop() override;