chore(assets): migrate local images to remote storage

- Replace local image paths with remote URLs in components, content, and config.
- Remove local image files from the public directory to reduce repository size.
- Add dark mode support for the hero background image in `index.vue`.

content/news/20251001-official-web-launch.md

@@ -4,14 +4,13 @@ date: "2025-10-01"
 updated: "2025-10-01"
 author: "麦祖奕学长"
 description: "永中校友会官网正式上线，为校友提供最新资讯、活动报名及互动交流平台。"
-cover: "/news/20251001-official-web-launch/Screenshot.png"
+cover: "https://img.yphsalumni.org/i/2025/11/28/qqc3ft.png"
 tags: ["活动", "公告", "产品更新"]
 category: "通知"
 highlight: true
 seoTitle: "永中校友会官网上线 | 最新活动与资讯平台"
 seoDescription: "永中校友会官网正式上线，校友可在平台获取最新资讯、报名活动及参与互动。"
-ogImage: "/images/og/news-launch.jpg"
-slug: "/news/20251001-official-web-launch"
+ogImage: "https://img.yphsalumni.org/i/2025/11/28/qqc3ft.png"
 ---
 永中校友会官网正式上线啦！🎉