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.cpp

@@ -9,6 +9,7 @@
 namespace {
 constexpr int PAGE_ITEMS = 23;
 constexpr int SKIP_PAGE_MS = 700;
+constexpr unsigned long GO_HOME_MS = 1000;
 }  // namespace
 
 void sortFileList(std::vector<std::string>& strs) {
@@ -53,7 +54,7 @@ void FileSelectionActivity::onEnter() {
 
   renderingMutex = xSemaphoreCreateMutex();
 
-  basepath = "/";
+  // basepath is set via constructor parameter (defaults to "/" if not specified)
   loadFiles();
   selectorIndex = 0;
 
@@ -83,6 +84,16 @@ void FileSelectionActivity::onExit() {
 }
 
 void FileSelectionActivity::loop() {
+  // Long press BACK (1s+) goes to root folder
+  if (inputManager.isPressed(InputManager::BTN_BACK) && inputManager.getHeldTime() >= GO_HOME_MS) {
+    if (basepath != "/") {
+      basepath = "/";
+      loadFiles();
+      updateRequired = true;
+    }
+    return;
+  }
+
   const bool prevReleased =
       inputManager.wasReleased(InputManager::BTN_UP) || inputManager.wasReleased(InputManager::BTN_LEFT);
   const bool nextReleased =
@@ -103,15 +114,17 @@ void FileSelectionActivity::loop() {
     } else {
       onSelect(basepath + files[selectorIndex]);
     }
-  } else if (inputManager.wasPressed(InputManager::BTN_BACK)) {
-    if (basepath != "/") {
-      basepath.replace(basepath.find_last_of('/'), std::string::npos, "");
-      if (basepath.empty()) basepath = "/";
-      loadFiles();
-      updateRequired = true;
-    } else {
-      // At root level, go back home
-      onGoHome();
+  } else if (inputManager.wasReleased(InputManager::BTN_BACK)) {
+    // Short press: go up one directory, or go home if at root
+    if (inputManager.getHeldTime() < GO_HOME_MS) {
+      if (basepath != "/") {
+        basepath.replace(basepath.find_last_of('/'), std::string::npos, "");
+        if (basepath.empty()) basepath = "/";
+        loadFiles();
+        updateRequired = true;
+      } else {
+        onGoHome();
+      }
     }
   } else if (prevReleased) {
     if (skipPage) {