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/app/admin/settings/BrandMediaPicker.tsx

@@ -88,7 +88,7 @@ export default function BrandMediaPicker({
               data-tip="用作草稿预览，点保存后才生效"
             >
               {/* eslint-disable-next-line @next/next/no-img-element */}
-              <img src={item.url} alt="" className="w-full h-full object-contain" loading="lazy" />
+              <img src={item.url} alt={`${SLOT_LABEL[slot]}候选图`} className="w-full h-full object-contain" loading="lazy" />
               <span
                 className="pointer-events-none absolute inset-0 flex items-center justify-center opacity-0 group-hover:opacity-100"
                 style={{ background: "color-mix(in srgb, var(--ink) 40%, transparent)", color: "#fff" }}