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

public/js/pages/settings-page.js

@@ -33,7 +33,7 @@ const SettingsPage = {
             <div class="form-group"><label>站点地址</label><input id="s-url" value="${U.esc(settings.site_url?.value||'')}"></div></div>
             <div class="grid-2"><div class="form-group"><label>页尾版权</label><input id="s-copyright" value="${U.esc(settings.copyright?.value||'')}" placeholder="&copy; 2024 MC举报系统"></div>
             <div class="form-group"><label>ICP备案号</label><input id="s-icp" value="${U.esc(settings.icp?.value||'')}" placeholder="沪ICP备XXXXXXXX号"></div></div>
-            <div class="form-group"><label>皮肤站地址</label><input id="s-skin-site" value="${U.esc(settings.skin_site?.value||'')}" placeholder="https://littleskin.cn（用于绑定皮肤站身份时自动查询UUID）"></div>
+            <div class="form-group"><label>皮肤站地址</label><input id="s-skin-site" value="${U.esc(settings.skin_site?.value||'')}" placeholder="https://littleskin.cn"></div>
             <button type="submit" class="btn btn-p btn-sm"><i class="fas fa-save"></i> 保存站点设置</button>
           </form>
         </div></div>