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/auth.js

@@ -118,13 +118,15 @@ router.post('/register', validateLengths({
     } catch {}
 
     const hashed = await bcrypt.hash(password, 10);
+    // 游戏UID为内置参数: 不接收玩家输入, 由系统自动生成
+    const builtinUid = 'U' + uuid().replace(/-/g, '').slice(0, 12).toUpperCase();
     const r = await query(
       'INSERT INTO users(username, password, email, game_name, game_uid, active, email_verified, source) VALUES (?,?,?,?,?,?,?,?)',
-      [username, hashed, email, game_name, game_uid||'', 1, 1, source || '']
+      [username, hashed, email, game_name, builtinUid, 1, 1, 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 {}
     res.status(201).json({ message: '注册成功，请登录' });
   } catch (err) {
@@ -233,7 +235,8 @@ router.post('/identities', authenticate, async (req, res) => {
     if (dupName) return res.status(400).json({ error: '该游戏名已绑定其他账号' });
 
     const r = await query('INSERT INTO user_identities(user_id, source, game_name, game_uid) VALUES (?,?,?,?)',
-      [req.user.id, source || '', game_name, game_uid || '']);
+      // 游戏UID为内置参数: 不接收玩家输入, 由系统自动生成
+      [req.user.id, source || '', game_name, 'U' + uuid().replace(/-/g, '').slice(0, 12).toUpperCase()]);
     res.status(201).json({ id: r.insertId, message: '绑定成功' });
   } catch (err) {
     console.error('[auth]', err);