fix: Fix dangling pointer (#1010)

## Summary

* **What is the goal of this PR?** Small fix for bug I found.

## Additional Context


1. `RecentBooksActivity::loop()` calls
`onSelectBook(recentBooks[selectorIndex].path)` - passing a
**reference** to the path
  2. `onSelectBook` is `onGoToReader` which first calls `exitActivity()`
3. `exitActivity()` triggers `RecentBooksActivity::onExit()` which call
`recentBooks.clear()`
4. The string reference `initialEpubPath` is now a **dangling
reference** - the underlying string has been destroyed
5. When the reference is then used in `new ReaderActivity(...)`, it
reads garbage memory
6. The same issue occurs in `HomeActivity` at line 200 with the same
pattern

The fix is to make a copy of the string in `onGoToReader` before calling
`exitActivity()`, so the path data persists even after the activity
clears its data structures.

---

### AI Usage

Did you use AI tools to help write this code? _**< YES >**_ Claude found
the bug, after I shared with it a serial log.

src/main.cpp

@@ -215,9 +215,9 @@ void onGoHome();
 void onGoToMyLibraryWithPath(const std::string& path);
 void onGoToRecentBooks();
 void onGoToReader(const std::string& initialEpubPath) {
+  const std::string bookPath = initialEpubPath;  // Copy before exitActivity() invalidates the reference
   exitActivity();
-  enterNewActivity(
-      new ReaderActivity(renderer, mappedInputManager, initialEpubPath, onGoHome, onGoToMyLibraryWithPath));
+  enterNewActivity(new ReaderActivity(renderer, mappedInputManager, bookPath, onGoHome, onGoToMyLibraryWithPath));
 }
 
 void onGoToFileTransfer() {