🔗 Add URL Content Extraction Feature (#514)

* feat: add URL content extraction for AI diagram generation

* Changes made as recommended by Claude:

1. Added a request timeout to prevent server resources from being tied up (route.ts)
2. Implemented runtime validation for the API response shape (url-utils.ts)
3. Removed hardcoded English error messages and replaced them with localized strings (url-input-dialog.tsx)
4. Fixed the incorrect i18n namespace (changed from pdf.* to url.*) (url-input-dialog.tsx and en/ja/zh.json)

* chore: restore package.json and package-lock.json

* fix: use i18n strings for URL dialog error messages

---------

Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

lib/i18n/dictionaries/ja.json

@@ -51,7 +51,8 @@
         "badResponse": "悪い応答",
         "clickToEdit": "クリックして編集",
         "editMessage": "メッセージを編集",
-        "saveAndSubmit": "保存して送信"
+        "saveAndSubmit": "保存して送信",
+        "ExtractURL": "URLから抽出"
     },
     "examples": {
         "title": "AI でダイアグラムを作成",
@@ -186,6 +187,15 @@
         "chars": "文字",
         "removeFile": "ファイルを削除"
     },
+    "url": {
+        "title": "URLからコンテンツを抽出",
+        "description": "URLを貼り付けてそのコンテンツを抽出および分析します",
+        "Extracting": "抽出中...",
+        "extract": "抽出",
+        "Cancel": "キャンセル",
+        "enterUrl": "URLを入力してください",
+        "invalidFormat": "無効なURL形式です"
+    },
     "reasoning": {
         "thinking": "考え中...",
         "thoughtFor": "{duration} 秒考えました",