公开页补齐自引用 Canonical 与按类型的结构化数据，图片补上说明文字。

后台设置和内容编辑改成可导航布局；部署用访客 origin 生成 Canonical，避免落到容器地址。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/markdown/MdImage.tsx

@@ -13,14 +13,14 @@ function collectGallery(from: HTMLImageElement): { items: LightboxItem[]; index:
   const items: LightboxItem[] = nodes
     .map((el) => ({
       src: el.currentSrc || el.getAttribute("src") || "",
-      alt: el.getAttribute("alt") || undefined,
+      alt: el.dataset.caption || undefined,
     }))
     .filter((it) => !!it.src);
   const selfSrc = from.currentSrc || from.getAttribute("src") || "";
   let index = items.findIndex((it) => it.src === selfSrc);
   if (index < 0) index = 0;
   if (items.length === 0 && selfSrc) {
-    return { items: [{ src: selfSrc, alt: from.alt || undefined }], index: 0 };
+    return { items: [{ src: selfSrc, alt: from.dataset.caption || undefined }], index: 0 };
   }
   return { items, index };
 }
@@ -39,10 +39,13 @@ export default function MdImage({ src, alt, title, className, ...rest }: MdImage
     setLightbox(gallery);
   }, []);
 
+  const provided = (typeof alt === "string" ? alt.trim() : "") || (typeof title === "string" ? title.trim() : "");
+  const imgAlt = provided || "图片";
+
   if (!src || broken) {
     return (
-      <span className="md-img-fallback" role="img" aria-label={alt || "图片加载失败"}>
-        {alt || "图片加载失败"}
+      <span className="md-img-fallback" role="img" aria-label={provided || "图片加载失败"}>
+        {provided || "图片加载失败"}
       </span>
     );
   }
@@ -53,7 +56,8 @@ export default function MdImage({ src, alt, title, className, ...rest }: MdImage
       <img
         {...rest}
         src={src}
-        alt={alt || ""}
+        alt={imgAlt}
+        data-caption={provided || undefined}
         title={title}
         className={`md-content-img${className ? ` ${className}` : ""}`}
         loading="lazy"