fix(admin): 修复设置页面 CSP style-src 违规导致加载失败

- 移除 site-settings 模板中的内联 style 属性（display:none, width:100px）
- 将内联样式迁移至 site-settings.css 的 CSS 类中
- site-settings-theme.js 用 className 替代 style.display 操作

templates/admin/html/site-settings/registration.html

@@ -46,7 +46,7 @@
                         <span class="setting-desc">强制阅读时的倒计时秒数</span>
                     </div>
                     <div class="setting-input-wrap">
-                        <input type="number" id="setting-guidelines-timer" class="setting-input" placeholder="60" min="1" style="width:100px" value="{{.GuidelinesTimer}}">
+                        <input type="number" id="setting-guidelines-timer" class="setting-input setting-input-narrow" placeholder="60" min="1" value="{{.GuidelinesTimer}}">
                         <button class="btn btn-sm btn-primary btn-save" data-key="registration.guidelines_timer" data-input="setting-guidelines-timer">保存</button>
                     </div>
                 </div>