fix: Handle UTF-8 BOM in config.json

Use utf-8-sig encoding to handle Windows BOM (Byte Order Mark)
that may be present in config.json files edited with Notepad.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/run_server.py

@@ -26,7 +26,8 @@ def load_config(config_path: str | None = None) -> dict:
         config_path = os.path.join(base_dir, "config.json")
 
     if os.path.exists(config_path):
-        with open(config_path, "r", encoding="utf-8") as f:
+        # Use utf-8-sig to handle Windows BOM (Byte Order Mark)
+        with open(config_path, "r", encoding="utf-8-sig") as f:
             return json.load(f)
     return {}