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/install.js

@@ -85,9 +85,11 @@ router.post('/complete', async (req, res) => {
     await initSchema(conn);
 
     const hashed = bcrypt.hashSync(admin_password, 10);
+    // 游戏UID为内置参数: 不接收输入, 由系统自动生成
+    const builtinUid = 'U' + crypto.randomBytes(6).toString('hex').toUpperCase();
     await conn.query(
       'INSERT INTO users(username, password, email, game_name, game_uid, role, active, email_verified) VALUES (?,?,?,?,?,?,1,1)',
-      [admin_username, hashed, admin_email, admin_game_name || admin_username, admin_game_uid || 'OWNER001', 'owner']
+      [admin_username, hashed, admin_email, admin_game_name || admin_username, builtinUid, 'owner']
     );
 
     const jwtSecret = crypto.randomBytes(32).toString('hex');