feat: add toolbar position setting and update email viewer layout

- Introduced a new setting for toolbar position in appearance settings, allowing users to choose between 'top' and 'below-subject'.
- Updated the EmailViewer component to conditionally render the toolbar based on the selected position.
- Enhanced the UI for the toolbar with action buttons for replying, archiving, and deleting emails.
- Added translations for the new toolbar position setting in multiple languages.

locales/ja/common.json

@@ -487,6 +487,12 @@
       "animations": {
         "label": "アニメーションを有効にする",
         "description": "スムーズな遷移とエフェクトを表示"
+      },
+      "toolbar_position": {
+        "label": "ツールバーの位置",
+        "description": "メールアクションボタン（返信、アーカイブ、削除など）の表示位置",
+        "top": "上部",
+        "below_subject": "件名の下"
       }
     },
     "keywords": {