全ページにSNS共有ボタンと見出しアンカーリンクを追加

- SocialShare コンポーネント: X/Facebook/LINE 共有ボタンを
  各ページフッターに表示
- Footer コンポーネントを上書きして SocialShare を組み込み
- 見出し（h2/h3）にホバーでコピー可能なアンカーリンクを追加
- custom.css にアンカーリンクのスタイルを追加

3 files changed, 163 insertions, 0 deletions

diff --git a/src/components/SocialShare.astro b/src/components/SocialShare.astro
new file mode 100644
index 0000000..a818822
--- /dev/null
+++ b/src/components/SocialShare.astro
@@ -0,0 +1,77 @@
+---
+export interface Props {
+  /** Page URL (defaults to current page) */
+  url?: string;
+  /** Page title */
+  title?: string;
+}
+
+const { url, title } = Astro.props;
+const pageUrl = url ?? Astro.url.href;
+const pageTitle = title ?? "";
+
+const encodedUrl = encodeURIComponent(pageUrl);
+const encodedTitle = encodeURIComponent(pageTitle);
+
+const SHARE_LINKS = [
+  {
+    label: "X",
+    href: `https://x.com/intent/tweet?url=${encodedUrl}&text=${encodedTitle}`,
+    svg: '<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M4 4l11.733 16h4.267l-11.733-16z"/><path d="M4 20l6.768-6.768m2.46-2.46L20 4"/></svg>',
+  },
+  {
+    label: "Facebook",
+    href: `https://www.facebook.com/sharer/sharer.php?u=${encodedUrl}`,
+    svg: '<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M18 2h-3a5 5 0 0 0-5 5v3H7v4h3v8h4v-8h3l1-4h-4V7a1 1 0 0 1 1-1h3z"/></svg>',
+  },
+  {
+    label: "LINE",
+    href: `https://social-plugins.line.me/lineit/share?url=${encodedUrl}`,
+    svg: '<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M2 12h2"/><path d="M6 8v8"/><path d="M10 6v12"/><path d="M14 9.5V20"/><path d="M18 6v12"/><path d="M22 8v8"/></svg>',
+  },
+];
+---
+
+<div class="social-share">
+  <span class="social-share-label">Share:</span>
+  {SHARE_LINKS.map((link) => (
+    <a
+      href={link.href}
+      target="_blank"
+      rel="noopener noreferrer"
+      class="social-share-btn"
+      title={`${link.label}で共有`}
+    >
+      <span set:html={link.svg} />
+    </a>
+  ))}
+</div>
+
+<style>
+  @layer starlight.core {
+    .social-share {
+      display: flex;
+      align-items: center;
+      gap: 0.5rem;
+      margin-top: 2rem;
+      padding-top: 1rem;
+      border-top: 1px solid var(--sl-color-gray-5);
+    }
+    .social-share-label {
+      font-size: 0.85rem;
+      color: var(--sl-color-gray-2);
+    }
+    .social-share-btn {
+      color: var(--sl-color-gray-2);
+      padding: 0.35rem;
+      border-radius: 4px;
+      transition: color 0.2s, background 0.2s;
+      display: flex;
+      align-items: center;
+    }
+    .social-share-btn:hover {
+      color: var(--sl-color-white);
+      background: var(--sl-color-gray-5);
+    }
+  }
+</style>
diff --git a/src/components/starlight/Footer.astro b/src/components/starlight/Footer.astro
new file mode 100644
index 0000000..7b10569
--- /dev/null
+++ b/src/components/starlight/Footer.astro
@@ -0,0 +1,64 @@
+---
+import EditLink from 'virtual:starlight/components/EditLink';
+import LastUpdated from 'virtual:starlight/components/LastUpdated';
+import Pagination from 'virtual:starlight/components/Pagination';
+import config from 'virtual:starlight/user-config';
+import { Icon } from '@astrojs/starlight/components';
+import SocialShare from '../SocialShare.astro';
+---
+
+<footer class="sl-flex">
+  <div class="meta sl-flex">
+    <EditLink />
+    <LastUpdated />
+  </div>
+  <Pagination />
+
+  <SocialShare />
+
+  {
+    config.credits && (
+      <a class="kudos sl-flex" href="https://starlight.astro.build">
+        <Icon name={'starlight'} /> {Astro.locals.t('builtWithStarlight.label')}
+      </a>
+    )
+  }
+</footer>
+
+<style>
+  @layer starlight.core {
+    footer {
+      flex-direction: column;
+      gap: 1.5rem;
+    }
+    .meta {
+      gap: 0.75rem 3rem;
+      justify-content: space-between;
+      flex-wrap: wrap;
+      margin-top: 3rem;
+      font-size: var(--sl-text-sm);
+      color: var(--sl-color-gray-3);
+    }
+    .meta > :global(p:only-child) {
+      margin-inline-start: auto;
+    }
+
+    .kudos {
+      align-items: center;
+      gap: 0.5em;
+      margin: 1.5rem auto;
+      font-size: var(--sl-text-xs);
+      text-decoration: none;
+      color: var(--sl-color-gray-3);
+    }
+    .kudos:hover {
+      color: var(--sl-color-white);
+    }
+  }
+
+  @layer starlight.components {
+    .kudos :global(svg) {
+      color: var(--sl-color-orange);
+    }
+  }
+</style>
diff --git a/src/styles/custom.css b/src/styles/custom.css
index 67a4c79..f765954 100644
--- a/src/styles/custom.css
+++ b/src/styles/custom.css
@@ -1048,3 +1048,25 @@ main:has(.home-hero) {
   font-size: 0.75em;
   vertical-align: super;
 }
+
+/* ── Heading Anchor Links ── */
+.heading-anchor {
+  opacity: 0;
+  margin-left: -1.2em;
+  padding-right: 0.3em;
+  color: var(--sl-color-gray-3);
+  text-decoration: none;
+  transition: opacity 0.2s;
+  float: left;
+}
+h2:hover .heading-anchor,
+h3:hover .heading-anchor,
+.heading-anchor.copied {
+  opacity: 1;
+}
+.heading-anchor:hover {
+  color: var(--sl-color-accent);
+}
+.heading-anchor.copied {
+  color: var(--sl-color-green-high, #16a34a);
+}