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.

docs/superpowers/plans/2026-05-10-article-i18n.md

@@ -369,8 +369,10 @@ return new class extends Migration
             DB::statement('ALTER TABLE documents DROP INDEX documents_search_index');
         }
 
-        // 5. Drop translatable columns from documents
+        // 5. Drop translatable columns from documents.
+        //    SQLite requires explicit dropIndex on the title index before dropColumn.
         Schema::table('documents', function (Blueprint $table) {
+            $table->dropIndex(['title']);
             $table->dropColumn(['title', 'content', 'rendered_html']);
         });
     }