Fix compilation issues

Fixed compilation errors:
- Added <dirent.h> include to file_system.h for DIR type on Linux
- Replaced xxx placeholder in chunk_server.c with actual chunks_dir path
  using state->store.path

src/file_system.h

@@ -3,6 +3,10 @@
 
 #include "basic.h"
 
+#ifndef _WIN32
+#include <dirent.h>
+#endif
+
 typedef struct {
     uint64_t data;
 } Handle;