fix: 统一所有 JSON 文件读写为 UTF-8 编码

Windows 中文环境默认编码为 GBK，未指定 encoding 会导致
config.json/all_keys.json 解析失败。修复 9 个文件共 17 处。

Closes #32

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

latency_test.py

@@ -15,7 +15,7 @@ DB_DIR = _cfg["db_dir"]
 KEYS_FILE = _cfg["keys_file"]
 DECRYPTED = os.path.join(_cfg["decrypted_dir"], "session", "session.db")
 
-with open(KEYS_FILE) as f:
+with open(KEYS_FILE, encoding="utf-8") as f:
     keys = json.load(f)
 enc_key = bytes.fromhex(keys["session/session.db"]["enc_key"])