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>

monitor_web.py

@@ -1883,7 +1883,7 @@ def main():
     print("  微信实时监听 (WAL增量 + SSE推送)", flush=True)
     print("=" * 60, flush=True)
 
-    with open(KEYS_FILE) as f:
+    with open(KEYS_FILE, encoding="utf-8") as f:
         keys = strip_key_metadata(json.load(f))
 
     session_key_info = get_key_info(keys, os.path.join("session", "session.db"))