fix: 补全 i18n 国际化，消除所有硬编码中文

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/i18n/zh-CN.json

@@ -1,12 +1,24 @@
 {
   "app": { "title": "五子棋 v2.0" },
+  "common": {
+    "back": "返回",
+    "back_to_menu": "返回菜单"
+  },
   "menu": {
     "local_game": "本地双人",
     "ai_game": "人机对战",
     "online_game": "网络对战",
     "online_game_disabled": "网络对战功能开发中",
     "load_replay": "加载棋谱",
-    "settings": "设置"
+    "settings": "设置",
+    "host_room": "创建房间",
+    "join_room": "加入房间",
+    "ip_placeholder": "IP:端口"
+  },
+  "ai_setup": {
+    "first_player": "先手",
+    "black_first": "黑棋 (先手)",
+    "white_second": "白棋 (后手)"
   },
   "game": {
     "black_turn": "黑棋回合",