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/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",