refactor: standardize error reporting on the log module

- Add log_perror() helper (context + strerror(errno)) to the log module
- Replace all bare perror() calls with log_perror() so errors are routed
  through the unified logger (stderr sink + optional --log-file sink)
- Convert fprintf(stderr, "Error:/Warning: ...") in client code to
  log_message(); raw fprintf kept only for progress/stats output

src/shared/log.c

@@ -1,6 +1,8 @@
 #include "log.h"
+#include <errno.h>
 #include <stdarg.h>
 #include <stdio.h>
+#include <string.h>
 #include <time.h>
 
 static const char* log_level_strings[] = {"DEBUG", "INFO", "WARN", "ERROR"};
@@ -50,3 +52,7 @@ void log_message(LogLevel log_level, const char* format, ...) {
     va_end(args);
   }
 }
+
+void log_perror(const char* context) {
+  log_message(LOG_LEVEL_ERROR, "%s: %s", context, strerror(errno));
+}