fix: debug printf of cover name (#690)

## Summary

Minor fix

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? no

src/activities/boot_sleep/SleepActivity.cpp

@@ -265,7 +265,7 @@ void SleepActivity::renderCoverSleepScreen() const {
   if (SdMan.openFileForRead("SLP", coverBmpPath, file)) {
     Bitmap bitmap(file);
     if (bitmap.parseHeaders() == BmpReaderError::Ok) {
-      Serial.printf("[SLP] Rendering sleep cover: %s\n", coverBmpPath);
+      Serial.printf("[SLP] Rendering sleep cover: %s\n", coverBmpPath.c_str());
       renderBitmapSleepScreen(bitmap);
       return;
     }