feat: chat language display, disabled EntitySelect items, dev scripts

Chat language:
- Added language_code field to TelegramChat model + migration
- Saved from message.from.language_code on webhook/polling
- Displayed as badge on bot chat cards and target receiver items
- Resolved from DB in target API response (works for existing receivers)
- Shown in chat picker dropdown (desc includes language)

EntitySelect improvements:
- Tracker-target link selector shows all targets, already-linked ones
  appear disabled with "Already linked" hint
- Receiver chat picker shows already-added chats as disabled

Dev scripts:
- scripts/restart-backend.sh and restart-frontend.sh
- Updated .claude/docs/dev-servers.md to reference scripts

frontend/src/lib/i18n/en.json

@@ -176,6 +176,7 @@
 		"linkedTargets": "targets",
 		"noLinkedTargets": "No targets linked. Add a target below.",
 		"addTarget": "Add target",
+		"alreadyLinked": "Already linked",
 		"testBasic": "Send test message",
 		"testPeriodic": "Test periodic summary",
 		"testScheduled": "Test scheduled assets",

frontend/src/lib/i18n/ru.json

@@ -176,6 +176,7 @@
 		"linkedTargets": "получатели",
 		"noLinkedTargets": "Нет привязанных получателей. Добавьте получателя ниже.",
 		"addTarget": "Добавить получателя",
+		"alreadyLinked": "Уже привязан",
 		"testBasic": "Отправить тестовое сообщение",
 		"testPeriodic": "Тест периодической сводки",
 		"testScheduled": "Тест запланированных фото",