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.cpp

@@ -14,6 +14,7 @@
 namespace {
 constexpr int pagesPerRefresh = 15;
 constexpr unsigned long skipChapterMs = 700;
+constexpr unsigned long goHomeMs = 1000;
 constexpr float lineCompression = 0.95f;
 constexpr int marginTop = 8;
 constexpr int marginRight = 10;
@@ -108,7 +109,14 @@ void EpubReaderActivity::loop() {
     xSemaphoreGive(renderingMutex);
   }
 
-  if (inputManager.wasPressed(InputManager::BTN_BACK)) {
+  // Long press BACK (1s+) goes directly to home
+  if (inputManager.isPressed(InputManager::BTN_BACK) && inputManager.getHeldTime() >= goHomeMs) {
+    onGoHome();
+    return;
+  }
+
+  // Short press BACK goes to file selection
+  if (inputManager.wasReleased(InputManager::BTN_BACK) && inputManager.getHeldTime() < goHomeMs) {
     onGoBack();
     return;
   }