feat: collapsible accordion slots for template editing UX

Template slot editors (notification + command) now use collapsible
accordion rows instead of showing all editors at once. Each slot
displays a compact header with status pill (empty/valid/warning/error).
Adds slot name filtering and a preview toggle button that swaps
between editor and rendered preview views.

frontend/src/lib/i18n/en.json

@@ -539,7 +539,8 @@
 		"assetFields": "Asset fields (in {% for asset in added_assets %})",
 		"albumFields": "Album fields (in {% for album in albums %})",
 		"confirmDelete": "Delete this template config?",
-		"invalidFormat": "Invalid format string"
+		"invalidFormat": "Invalid format string",
+		"filterSlots": "Filter slots..."
 	},
 	"templateVars": {
 		"message_assets_added": {

frontend/src/lib/i18n/ru.json

@@ -539,7 +539,8 @@
 		"assetFields": "Поля файла (в {% for asset in added_assets %})",
 		"albumFields": "Поля альбома (в {% for album in albums %})",
 		"confirmDelete": "Удалить эту конфигурацию шаблона?",
-		"invalidFormat": "Некорректная строка формата"
+		"invalidFormat": "Некорректная строка формата",
+		"filterSlots": "Фильтр слотов..."
 	},
 	"templateVars": {
 		"message_assets_added": {