Fix file browser navigation for non-ASCII folder names (#178)

## Summary

* **What is the goal of this PR?**

Fix file browser failing to navigate into subdirectories with non-ASCII
(Korean/Unicode) folder names.

* **What changes are included?**

- Enable UTF-8 long file names in SdFat (`USE_UTF8_LONG_NAMES=1`)
- Add directory validation before iterating files
- Add `rewindDirectory()` call for stability

## Additional Context

platformio.ini

@@ -26,6 +26,8 @@ build_flags =
   -DXML_GE=0
   -DXML_CONTEXT_BYTES=1024
   -std=c++2a
+# Enable UTF-8 long file names in SdFat
+  -DUSE_UTF8_LONG_NAMES=1
 
 ; Board configuration
 board_build.flash_mode = dio