Add documents.delete_translation lang key for editor button

The editor's delete-translation button used `__('messages.documents.delete_translation') ?? __('messages.documents.delete')`, but `__()` returns the key string (not null) on miss so the `??` fallback never fires — the button rendered the literal key. Adds the missing key to all 16 locales (en+ja human-translated, others mirror en) and simplifies the blade to a single `__()` call.

Plan doc also reflects the SQLite dropIndex requirement found during Task 2.

src/lang/ja/messages.php

@@ -44,6 +44,7 @@
         'fallback_notice' => 'この記事には選択した言語の翻訳がありません。元の言語版を表示しています。',
         'add_translation' => '翻訳を追加',
         'set_as_default' => 'デフォルトに設定',
+        'delete_translation' => '翻訳を削除',
         'delete_translation_blocked' => 'デフォルト言語の翻訳は削除できません。',
         'translation_tabs_label' => '言語',
     ],