fix: 处理配置文件中的 UTF-8 BOM 编码问题 (#5376)

* fix(config): handle UTF-8 BOM in configuration file loading

Problem:
On Windows, some text editors (like Notepad) automatically add UTF-8 BOM
to JSON files when saving. This causes json.decoder.JSONDecodeError:
"Unexpected UTF-8 BOM" and AstrBot fails to start when cmd_config.json
contains BOM.

Solution:
Add defensive check to strip UTF-8 BOM (\ufeff) if present before
parsing JSON configuration file.

Impact:
- Improves robustness and cross-platform compatibility
- No breaking changes to existing functionality
- Fixes startup failure when configuration file has UTF-8 BOM encoding

Relates-to: Windows editor compatibility issues

* style: fix code formatting with ruff

Fix single quote to double quote to comply with project code style.

astrbot/core/config/astrbot_config.py

@@ -52,6 +52,9 @@ class AstrBotConfig(dict):
 
         with open(config_path, encoding="utf-8-sig") as f:
             conf_str = f.read()
+            # Handle UTF-8 BOM if present
+            if conf_str.startswith("\ufeff"):
+                conf_str = conf_str[1:]
             conf = json.loads(conf_str)
 
         # 检查配置完整性，并插入