Add CreateTemplate endpoint and related functionality for channels without grids

Implement a new API endpoint to create a template for channels lacking a grid, enhancing the template management capabilities. Update frontend components to support the creation of templates directly from the channel detail view, including user feedback and translations for improved clarity. Add integration tests to ensure the new functionality works as expected.

frontend/src/shared/lib/i18n.ts

@@ -398,6 +398,9 @@ const resources = {
             air: 'Эфир',
           },
           noTemplate: 'Сетка канала не загрузилась',
+          createTemplate: 'Создать сетку',
+          createTemplateHint:
+            'Появится пустая сетка с фоновым слоем — дальше добавляйте слои и слоты.',
           rules: 'Правила отбора',
           rulesHint:
             'Жёсткие фильтры: отсекают неподходящее до жребия. Как и правка сетки, эфир не двигают — нужно применить.',
@@ -1100,6 +1103,9 @@ const resources = {
             air: 'On air',
           },
           noTemplate: 'The channel grid failed to load',
+          createTemplate: 'Create the grid',
+          createTemplateHint:
+            'An empty grid with a background layer appears — then add layers and slots.',
           rules: 'Candidate rules',
           rulesHint:
             'Hard filters: they cut out what is not allowed before the draw. Like grid edits, they do not move the air — apply to take effect.',