fix: correct alt attribute for image tag

- Update alt attribute to reference the first item's image_alt
- This change improves accessibility by ensuring the correct
  descriptive text is used if the latest_arts is present.

Prior to this change, there was a risk of displaying an incorrect
description if the latest_arts was empty, which could lead to
confusion for users relying on alt text for context.

app/views/home/index.html.erb

@@ -5,7 +5,7 @@
       <div class="absolute inset-0">
         <% begin %>
           <%= image_tag @latest_arts&.first&.webp_image&.processed,
-                        alt: @latest_arts.image_alt,
+                        alt: @latest_arts&.first&.image_alt,
                         class: "w-full h-full object-cover" %>
         <% rescue StandardError => e %>
           <% Rails.logger.error("图片加载失败: #{e.message}") if defined?(Rails) %>