fix(pagination): align the load-more skeleton width with the cards

首页「加载更多」的骨架比列表本身宽出一截：.posts-row 用
margin-left/right:-8px 抵消卡片的水平边距，而卡片边距规则是
`.posts-row > *:not(.post_ajax_loader)`，把骨架排除在补偿链之外，
于是骨架左右各宽出 8px（合计 16px）。移动端的 5px 覆盖块同理。

小工具的 .widget-ajaxpager / tab pane 也是一条独立的负边距补偿链
（margin: -8px 抵消 .posts-item 的 8px），骨架同样溢出。

修法是给骨架补上与卡片等同的水平边距，而不是写 inline display
（会盖过 .is-loading 类规则）。改了 main.css、post-card.css 的
8px / 5px 两组规则，用户中心面板另加一条归零规则，因为那里的
容器 margin 与卡片边距都已清零，不能再叠加补偿。

顺带补一条契约测试：删掉任一 gutter 规则即失败，防止回归。

Verification:
- node --test tests/**/*.test.cjs  471 passed / 0 failed
- php tests/widget-system/run.php  136 assertions
- php tools/audit-widget-system.php
- php tools/audit-admin-contracts.php  errors=0 (56 既有 baseline 警告)
- node tools/build-public-assets.mjs  CSS/JS 产物已重建

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/css/main.min.css

@@ -1 +1 @@
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+body{--mian-max-width: 1200px;--posts-card-scale: 70%;--posts-list-scale: 70%;--theme-color: #f04494;--focus-color: var(--theme-color);--focus-shadow-color: rgba(253, 83, 161, .4);--key-color: #333;--main-color: #4e5358;--main-shadow: rgba(116, 116, 116, .08);--muted-color: #777;--muted-2-color: #999;--muted-3-color: #b1b1b1;--muted-4-color: #d2d2d2;--body-bg-color: #f5f6f7;--main-bg-color: #fff;--muted-bg-color: #eee;--muted-bg-2-color: #f6f7f8;--main-border-color: rgba(50, 50, 50, .06);--muted-border-color: rgba(0, 0, 0, .03);--main-radius: 8px;--mini-radius: calc(var(--main-radius) / 1.4);--blur-bg: rgba(255, 255, 255, .8);--float-btn-bg: rgba(200, 200, 200, .4);--header-bg: var(--blur-bg);--header-color: var(--main-color);--footer-bg: var(--main-bg-color);--footer-color: var(--muted-2-color)}.dark-theme{--key-color: #f8fafc;--main-color: #e5eef7;--main-shadow: rgba(24, 24, 24, .1);--muted-color: #b4b6bb;--muted-2-color: #888a8f;--muted-3-color: #636469;--muted-4-color: #43454a;--body-bg-color: #292a2d;--main-bg-color: #323335;--muted-bg-color: #2d2e31;--muted-bg-2-color: #37383a;--main-border-color: rgba(114, 114, 114, .1);--muted-border-color: rgba(184, 184, 184, .02);--blur-bg: rgba(50, 51, 53, .8);--float-btn-bg: rgba(70, 70, 70, .4);--header-bg: var(--blur-bg);--header-color: var(--main-color);--footer-bg: var(--main-bg-color);--footer-color: var(--muted-2-color)}::view-transition-old(root),::view-transition-new(root){animation:none;mix-blend-mode:normal}.dark::view-transition-old(root){z-index:1}.dark::view-transition-new(root){z-index:2147483646}::view-transition-old(root){z-index:2147483646}::view-transition-new(root){z-index:1}@supports not ((-webkit-backdrop-filter: blur(20px)) or (backdrop-filter: blur(20px))){body{--blur-bg: #ffffff}body.dark-theme{--blur-bg: #37383a}}body:has(.modal.in),body.modal-open{--blur-bg: #ffffff}body.dark-theme:has(.modal.in),body.dark-theme.modal-open{--blur-bg: #37383a}body.fps-accelerat{--blur-bg: #ffffff}body.fps-accelerat.dark-theme{--blur-bg: #37383a}body.fps-accelerat *{-webkit-backdrop-filter:unset!important;backdrop-filter:unset!important}a[href],.relates-thumb li a>.post-info,.more-posts a>span{transition:color .2s,background .3s,opacity .3s,box-shadow .3s,transform .3s}.main-shadow{transition:box-shadow .3s}.but:focus,.but:hover,.progress,.navbar-nav>li:before{transition:.3s}.navbar-top .sub-menu,.theme-popover,.hover-show .hover-show-con{opacity:0;transform:translateY(6px);transition:.4s}.navbar-top li:hover>.sub-menu,.hover-show:hover .hover-show-con,.hover-show:hover~.hover-show-con,.hover-show:hover~.sub-menu{opacity:1;transform:translateY(0)}.hover-show{position:relative;cursor:pointer;z-index:10}.hover-show .hover-show-con{position:absolute;visibility:hidden}.hover-show:hover .hover-show-con,.hover-show:hover~.hover-show-con,.hover-show:hover~.sub-menu{visibility:unset}.match-ok,.navbar-top .sub-menu li>a,.ic-close{transition:.3s}.ic-close:hover{transform:rotate(270deg);color:inherit}.social_loginbar a,.item-thumbnail img,.dosc-nav .fa{transition:.2s ease-out}.hover-zoom:hover,.hover-zoom-img:hover img,.social_loginbar .xh-social-item:hover,.social_loginbar .social-login-item:hover{transform:scale(1.05)}.hover-zoom-sm:hover,.hover-zoom-img-sm:hover img,.posts-mini:hover img,.posts-item:hover .item-thumbnail img,.posts-item.mult-thumb .thumb-items>span>img:hover{transform:scale(1.02)}.main-min-height{min-height:calc(100vh - 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