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/components/menu/AiGameSetup.tsx

@@ -42,10 +42,10 @@ export default function AiGameSetup({ onBack, onStart }: Props) {
         </select>
       </label>
       <label>
-        先手:
+        {t('ai_setup.first_player')}:
         <select value={playerColor} onChange={(e) => setPlayerColor(e.target.value as Color)}>
-          <option value="Black">黑棋 (先手)</option>
-          <option value="White">白棋 (后手)</option>
+          <option value="Black">{t('ai_setup.black_first')}</option>
+          <option value="White">{t('ai_setup.white_second')}</option>
         </select>
       </label>
       <label>
@@ -54,7 +54,7 @@ export default function AiGameSetup({ onBack, onStart }: Props) {
       </label>
       <div className="setup-actions">
         <button onClick={handleStart}>{t('game.new_game')}</button>
-        <button onClick={onBack}>返回</button>
+        <button onClick={onBack}>{t('common.back')}</button>
       </div>
     </div>
   );

src/components/menu/LoadReplay.tsx

@@ -38,7 +38,7 @@ export default function LoadReplay({ onBack, onStart }: Props) {
     <div className="setup-panel">
       <h2>{t('menu.load_replay')}</h2>
       <input ref={fileRef} type="file" accept=".json" onChange={handleFile} />
-      <button onClick={onBack} style={{ marginTop: 12 }}>返回</button>
+      <button onClick={onBack} style={{ marginTop: 12 }}>{t('common.back')}</button>
     </div>
   );
 }

src/components/menu/LocalGameSetup.tsx

@@ -31,7 +31,7 @@ export default function LocalGameSetup({ onBack, onStart }: Props) {
       <h2>{t('menu.local_game')}</h2>
       <div className="setup-actions">
         <button onClick={handleStart}>{t('game.new_game')}</button>
-        <button onClick={onBack}>返回</button>
+        <button onClick={onBack}>{t('common.back')}</button>
       </div>
     </div>
   );

src/components/menu/OnlineSetup.tsx

@@ -29,12 +29,12 @@ export default function OnlineSetup({ onBack, onStart }: Props) {
   return (
     <div className="setup-panel">
       <h2>{t('menu.online_game')}</h2>
-      <button onClick={handleHost}>创建房间</button>
+      <button onClick={handleHost}>{t('menu.host_room')}</button>
       <div style={{ display: 'flex', gap: 8, marginTop: 12 }}>
-        <input value={ip} onChange={(e) => setIp(e.target.value)} placeholder="IP:端口" />
-        <button onClick={handleJoin} disabled={!ip}>加入房间</button>
+        <input value={ip} onChange={(e) => setIp(e.target.value)} placeholder={t('menu.ip_placeholder') as string} />
+        <button onClick={handleJoin} disabled={!ip}>{t('menu.join_room')}</button>
       </div>
-      <button onClick={onBack} style={{ marginTop: 12 }}>返回</button>
+      <button onClick={onBack} style={{ marginTop: 12 }}>{t('common.back')}</button>
     </div>
   );
 }

src/components/replay/ReplayView.tsx

@@ -41,7 +41,7 @@ export default function ReplayView({ onBackToMenu }: Props) {
         onPrev={() => setReplayStep(Math.max(0, step - 1))}
         onNext={() => setReplayStep(Math.min(moves.length, step + 1))}
       />
-      <button onClick={onBackToMenu}>返回菜单</button>
+      <button onClick={onBackToMenu}>{t('common.back_to_menu')}</button>
     </div>
   );
 }

src/i18n/en.json

@@ -1,12 +1,24 @@
 {
   "app": { "title": "Gobang v2.0" },
+  "common": {
+    "back": "Back",
+    "back_to_menu": "Back to Menu"
+  },
   "menu": {
     "local_game": "Local 2-Player",
     "ai_game": "VS AI",
     "online_game": "Online",
     "online_game_disabled": "Online mode is under development",
     "load_replay": "Load Replay",
-    "settings": "Settings"
+    "settings": "Settings",
+    "host_room": "Create Room",
+    "join_room": "Join Room",
+    "ip_placeholder": "IP:Port"
+  },
+  "ai_setup": {
+    "first_player": "First Player",
+    "black_first": "Black (First)",
+    "white_second": "White (Second)"
   },
   "game": {
     "black_turn": "Black's Turn",

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",