feat(settings): protect model provider deletion and cascade credential updates

- Block deletion of a model provider when it is referenced by any agent,
  returning an error that lists the agent names so the user knows what to unlink first
- When a provider's api_url or api_key is updated, automatically propagate
  the new credentials to all dependent agents: updates env_json in the database
  and patches on-disk config files (Claude Code settings.json, Gemini settings.json,
  Codex auth.json + config.toml, OpenCode auth.json) using the same field names
  and structure as the agent settings UI
- Fix error message display in provider dialogs for both Tauri and web transports,
  which throw plain objects rather than Error instances

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/i18n/messages/ar.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "تم حذف المزود.",
     "deleteConfirmTitle": "حذف المزود",
     "deleteConfirmMessage": "سيتم حذف المزود \"{name}\" نهائياً. هل أنت متأكد؟",
+    "deleteBlockedByAgent": "{agents} يستخدم هذا المزود. يرجى إلغاء الربط قبل الحذف.",
     "cancel": "إلغاء",
     "delete": "حذف",
     "create": "إنشاء",

src/i18n/messages/de.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "Anbieter gelöscht.",
     "deleteConfirmTitle": "Anbieter löschen",
     "deleteConfirmMessage": "Der Anbieter \"{name}\" wird dauerhaft gelöscht. Sind Sie sicher?",
+    "deleteBlockedByAgent": "{agents} verwendet diesen Anbieter. Bitte trennen Sie die Verbindung vor dem Löschen.",
     "cancel": "Abbrechen",
     "delete": "Löschen",
     "create": "Erstellen",

src/i18n/messages/en.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "Provider deleted.",
     "deleteConfirmTitle": "Delete Provider",
     "deleteConfirmMessage": "This will permanently delete the provider \"{name}\". Are you sure?",
+    "deleteBlockedByAgent": "{agents} is currently using this provider. Please unlink before deleting.",
     "cancel": "Cancel",
     "delete": "Delete",
     "create": "Create",

src/i18n/messages/es.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "Proveedor eliminado.",
     "deleteConfirmTitle": "Eliminar Proveedor",
     "deleteConfirmMessage": "Esto eliminará permanentemente el proveedor \"{name}\". ¿Está seguro?",
+    "deleteBlockedByAgent": "{agents} está usando este proveedor. Desvincúlelo antes de eliminarlo.",
     "cancel": "Cancelar",
     "delete": "Eliminar",
     "create": "Crear",

src/i18n/messages/fr.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "Fournisseur supprimé.",
     "deleteConfirmTitle": "Supprimer le Fournisseur",
     "deleteConfirmMessage": "Le fournisseur \"{name}\" sera définitivement supprimé. Êtes-vous sûr ?",
+    "deleteBlockedByAgent": "{agents} utilise ce fournisseur. Veuillez le dissocier avant de supprimer.",
     "cancel": "Annuler",
     "delete": "Supprimer",
     "create": "Créer",

src/i18n/messages/ja.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "プロバイダーを削除しました。",
     "deleteConfirmTitle": "プロバイダーを削除",
     "deleteConfirmMessage": "プロバイダー「{name}」を完全に削除しますか？",
+    "deleteBlockedByAgent": "{agents} がこのプロバイダーを使用中です。削除する前にリンクを解除してください。",
     "cancel": "キャンセル",
     "delete": "削除",
     "create": "作成",

src/i18n/messages/ko.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "제공업체가 삭제되었습니다.",
     "deleteConfirmTitle": "제공업체 삭제",
     "deleteConfirmMessage": "제공업체 \"{name}\"을(를) 영구적으로 삭제하시겠습니까?",
+    "deleteBlockedByAgent": "{agents}이(가) 이 공급자를 사용 중입니다. 삭제하기 전에 연결을 해제해 주세요.",
     "cancel": "취소",
     "delete": "삭제",
     "create": "생성",

src/i18n/messages/pt.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "Provedor excluído.",
     "deleteConfirmTitle": "Excluir Provedor",
     "deleteConfirmMessage": "O provedor \"{name}\" será excluído permanentemente. Tem certeza?",
+    "deleteBlockedByAgent": "{agents} está usando este provedor. Desvincule-o antes de excluir.",
     "cancel": "Cancelar",
     "delete": "Excluir",
     "create": "Criar",

src/i18n/messages/zh-CN.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "供应商已删除。",
     "deleteConfirmTitle": "删除供应商",
     "deleteConfirmMessage": "确定要永久删除供应商「{name}」吗？",
+    "deleteBlockedByAgent": "{agents} 正在使用该配置，请先解除关联后再删除。",
     "cancel": "取消",
     "delete": "删除",
     "create": "创建",

src/i18n/messages/zh-TW.json

@@ -1849,6 +1849,7 @@
     "deleteSuccess": "供應商已刪除。",
     "deleteConfirmTitle": "刪除供應商",
     "deleteConfirmMessage": "確定要永久刪除供應商「{name}」嗎？",
+    "deleteBlockedByAgent": "{agents} 正在使用此設定，請先解除關聯後再刪除。",
     "cancel": "取消",
     "delete": "刪除",
     "create": "建立",