Add last update date display from Git commit history

- Create git utility to fetch last commit timestamp
- Display date in Japanese format (YYYY年M月D日)
- Show last update date alongside recipe statistics

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: kaisugi

src/components/Stats.astro

@@ -1,10 +1,13 @@
 ---
 import { recipes, getAllIngredients } from '../data/recipes';
+import { getLastUpdateDate } from '../utils/git';
 
 const totalDishes = recipes.length;
 const totalIngredients = getAllIngredients(recipes).length;
+const lastUpdated = getLastUpdateDate();
 ---
 
 <div class="mt-8 pt-6 border-t border-gray-200 text-center text-sm text-gray-500">
-  全{totalDishes}品 / {totalIngredients}種類の食材
+  <div>全{totalDishes}品 / {totalIngredients}種類の食材</div>
+  <div class="mt-1">最終更新: {lastUpdated}</div>
 </div>

src/utils/git.ts

@@ -0,0 +1,34 @@
+import { execSync } from 'child_process';
+
+/**
+ * リポジトリの最終更新日を日本語形式で取得
+ * @returns 日本語形式の日付（例: "2025年12月25日"）、取得失敗時は "不明"
+ */
+export function getLastUpdateDate(): string {
+  try {
+    // Git コミットのタイムスタンプを取得（Unix timestamp）
+    const timestampStr = execSync('git log -1 --format=%ct', {
+      encoding: 'utf8',
+      cwd: process.cwd(),
+    }).trim();
+
+    const timestamp = parseInt(timestampStr, 10);
+
+    if (isNaN(timestamp)) {
+      return '不明';
+    }
+
+    // Unix timestampをミリ秒に変換してDateオブジェクト化
+    const date = new Date(timestamp * 1000);
+
+    // 日本語形式にフォーマット
+    const year = date.getFullYear();
+    const month = date.getMonth() + 1;
+    const day = date.getDate();
+
+    return `${year}年${month}月${day}日`;
+  } catch (error) {
+    console.warn('Failed to get last update date from Git:', error);
+    return '不明';
+  }
+}