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/main.cpp

@@ -142,6 +142,7 @@ void onGoToReader(const std::string& initialEpubPath) {
   enterNewActivity(new ReaderActivity(renderer, inputManager, initialEpubPath, onGoHome));
 }
 void onGoToReaderHome() { onGoToReader(std::string()); }
+void onContinueReading() { onGoToReader(APP_STATE.openEpubPath); }
 
 void onGoToFileTransfer() {
   exitActivity();
@@ -155,7 +156,8 @@ void onGoToSettings() {
 
 void onGoHome() {
   exitActivity();
-  enterNewActivity(new HomeActivity(renderer, inputManager, onGoToReaderHome, onGoToSettings, onGoToFileTransfer));
+  enterNewActivity(new HomeActivity(renderer, inputManager, onContinueReading, onGoToReaderHome, onGoToSettings,
+                                    onGoToFileTransfer));
 }
 
 void setupDisplayAndFonts() {