feat(settings): add iframe CSP warning for home content

Add a warning message to inform admins that some websites may have
X-Frame-Options or CSP policies that prevent iframe embedding.

frontend/src/i18n/locales/en.ts

@@ -1903,7 +1903,8 @@ export default {
         logoReadError: 'Failed to read the image file',
         homeContent: 'Home Page Content',
         homeContentPlaceholder: 'Enter custom content for the home page. Supports Markdown & HTML. If a URL is entered, it will be displayed as an iframe.',
-        homeContentHint: 'Customize the home page content. Supports Markdown/HTML. If you enter a URL (starting with http:// or https://), it will be used as an iframe src to embed an external page. When set, the default status information will no longer be displayed.'
+        homeContentHint: 'Customize the home page content. Supports Markdown/HTML. If you enter a URL (starting with http:// or https://), it will be used as an iframe src to embed an external page. When set, the default status information will no longer be displayed.',
+        homeContentIframeWarning: '⚠️ iframe mode note: Some websites have X-Frame-Options or CSP security policies that prevent embedding in iframes. If the page appears blank or shows an error, please verify the target website allows embedding, or consider using HTML mode to build your own content.'
       },
       smtp: {
         title: 'SMTP Settings',

frontend/src/i18n/locales/zh.ts

@@ -2046,7 +2046,8 @@ export default {
         logoReadError: '读取图片文件失败',
         homeContent: '首页内容',
         homeContentPlaceholder: '在此输入首页内容，支持 Markdown & HTML 代码。如果输入的是一个链接，则会使用该链接作为 iframe 的 src 属性。',
-        homeContentHint: '自定义首页内容，支持 Markdown/HTML。如果输入的是链接（以 http:// 或 https:// 开头），则会使用该链接作为 iframe 的 src 属性，这允许你设置任意网页作为首页。设置后首页的状态信息将不再显示。'
+        homeContentHint: '自定义首页内容，支持 Markdown/HTML。如果输入的是链接（以 http:// 或 https:// 开头），则会使用该链接作为 iframe 的 src 属性，这允许你设置任意网页作为首页。设置后首页的状态信息将不再显示。',
+        homeContentIframeWarning: '⚠️ iframe 模式提示：部分网站设置了 X-Frame-Options 或 CSP 安全策略，禁止被嵌入到 iframe 中。如果页面显示空白或报错，请确认目标网站允许被嵌入，或考虑使用 HTML 模式自行构建页面内容。'
       },
       smtp: {
         title: 'SMTP 设置',

frontend/src/views/admin/SettingsView.vue

@@ -577,6 +577,10 @@
               <p class="mt-1.5 text-xs text-gray-500 dark:text-gray-400">
                 {{ t('admin.settings.site.homeContentHint') }}
               </p>
+              <!-- iframe CSP Warning -->
+              <p class="mt-2 text-xs text-amber-600 dark:text-amber-400">
+                {{ t('admin.settings.site.homeContentIframeWarning') }}
+              </p>
             </div>
           </div>
         </div>