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/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",