feat: game UID fully internal + remove multi-identity copy

- game_uid is now an internal param: auto-generated server-side
  (U + 12 hex) on register/bind-identity/install/admin-create/external
  register; player-supplied game_uid ignored
- Removed all player-facing UID inputs: register page, install page,
  identity bind modal, ticket-create 'your UID' field
- Removed player-facing UID display: identity table column,
  ticket detail/tracking reporter UID
- Removed '同一账号可绑定网易端 + 皮肤站身份' / '可绑定多个来源身份' copy
- Kept target_game_uid (reported player) and admin panel UID management
- settings-page skin-site placeholder updated

backend/routes/external.js

@@ -166,14 +166,16 @@ router.post('/auth/register', async (req, res) => {
 
     const hashed = bcrypt.hashSync(password, 10);
     const verifyToken = uuid();
-    const r = await query('INSERT INTO users(username,password,email,game_name,game_uid,verify_token,verify_expires,source) VALUES (?,?,?,?,?,?,DATE_ADD(NOW(), INTERVAL 24 HOUR),?)', [username, hashed, email, game_name, game_uid || '', verifyToken, source || '']);
+    // 游戏UID为内置参数: 不接收客户端输入, 由系统自动生成
+    const builtinUid = 'U' + crypto.randomBytes(6).toString('hex').toUpperCase();
+    const r = await query('INSERT INTO users(username,password,email,game_name,game_uid,verify_token,verify_expires,source) VALUES (?,?,?,?,?,?,DATE_ADD(NOW(), INTERVAL 24 HOUR),?)', [username, hashed, email, game_name, builtinUid, verifyToken, source || '']);
     try {
       await query('INSERT INTO user_identities(user_id, source, game_name, game_uid) VALUES (?,?,?,?)',
-        [r.insertId, source || '', game_name, game_uid || '']);
+        [r.insertId, source || '', game_name, builtinUid]);
     } catch {}
 
     const site = await getRow("SELECT v FROM settings WHERE k='site_url'");
-    const sent = await sendEmail(email, 'verify_email', { username, game_name, game_uid, verify_link: `${site?.v||'http://localhost:3100'}#/verify?token=${verifyToken}` });
+    const sent = await sendEmail(email, 'verify_email', { username, game_name, game_uid: builtinUid, verify_link: `${site?.v||'http://localhost:3100'}#/verify?token=${verifyToken}` });
 
     if (!sent) {
       await query('UPDATE users SET email_verified=1,active=1,verify_token=NULL WHERE username=?', [username]);