Add external article link support to blog list

[Copilot]

Enables blog list to display links to articles published on external platforms (Zenn, Qiita, etc).

Changes

• Schema: Added optional externalUrl field to blog collection
• BlogCard component:
  • Routes to external URL when externalUrl is present
  • Renders external link icon and "外部記事" badge for visual distinction
  • Opens external links in new tab with rel="noopener noreferrer"

Usage

Add externalUrl to blog post frontmatter:

---
title: My Article on Zenn
pubDate: '2024-12-01'
author: Naoki
tags: [Tech]
externalUrl: https://zenn.dev/username/articles/my-article
---

Screenshot

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• fonts.googleapis.com
  • Triggering command: /usr/local/bin/node node /home/REDACTED/work/web/web/node_modules/.bin/../astro/astro.js build (dns block)
• telemetry.astro.build
  • Triggering command: /usr/local/bin/node node /home/REDACTED/work/web/web/node_modules/.bin/../astro/astro.js build (dns block)
  • Triggering command: /usr/local/bin/node node /home/REDACTED/work/web/web/node_modules/.bin/../astro/astro.js dev (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ブログ一覧に外部記事へのリンクを設置可能にする</issue_title>
<issue_description>ブログ一覧に、外部サイトで執筆した記事へのリンクを設定して表示できるようにする。</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ブログ一覧に外部記事へのリンクを設置可能にする #72

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