fix: Fix a dangling pointer (#939)

## Summary

* **What is the goal of this PR?** Fix a dangling pointer issue caused
by using `.c_str()` on a temporary `std::string`.

`basepath.substr()` creates a temporary `std::string`, and calling
`.c_str()` on it returns a pointer to its internal buffer (not a copy).
Since the temporary string is destroyed at the end of the full
expression, `folderName` ends up holding a dangling pointer, leading to
undefined behavior.

To solve this, we stores the result in a persistent `std::string`
object, ensuring the underlying buffer remains valid for the duration of
its use.

A similar pattern caused the behavior reported in
https://github.com/crosspoint-reader/crosspoint-reader/pull/728#issuecomment-3902529697

---

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src/activities/home/MyLibraryActivity.cpp

@@ -196,8 +196,8 @@ void MyLibraryActivity::render(Activity::RenderLock&&) {
   const auto pageHeight = renderer.getScreenHeight();
   auto metrics = UITheme::getInstance().getMetrics();
 
-  auto folderName = basepath == "/" ? tr(STR_SD_CARD) : basepath.substr(basepath.rfind('/') + 1).c_str();
-  GUI.drawHeader(renderer, Rect{0, metrics.topPadding, pageWidth, metrics.headerHeight}, folderName);
+  std::string folderName = (basepath == "/") ? tr(STR_SD_CARD) : basepath.substr(basepath.rfind('/') + 1);
+  GUI.drawHeader(renderer, Rect{0, metrics.topPadding, pageWidth, metrics.headerHeight}, folderName.c_str());
 
   const int contentTop = metrics.topPadding + metrics.headerHeight + metrics.verticalSpacing;
   const int contentHeight = pageHeight - contentTop - metrics.buttonHintsHeight - metrics.verticalSpacing;