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`.

nuxt.config.ts

@@ -51,7 +51,7 @@ export default defineNuxtConfig({
         { property: "og:url", content: "https://yphsalumni.org" }, // ✅ 换成你网站的真实域名
         {
           property: "og:image",
-          content: "https://yphsalumni.org/hero-image-2.jpg?v=2",
+          content: "https://img.yphsalumni.org/i/2025/11/28/qk9fe8.png",
         }, // ✅ 上传一张封面图
 
         // Twitter Card
@@ -63,7 +63,7 @@ export default defineNuxtConfig({
         },
         {
           name: "twitter:image",
-          content: "https://yphsalumni.org/hero-image-2.jpg?v=2",
+          content: "https://img.yphsalumni.org/i/2025/11/28/qk9fe8.png",
         },
       ],
     },