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-tauri/src/web/handlers/model_provider.rs

@@ -74,6 +74,7 @@ pub async fn update_model_provider(
         params.api_url,
         params.api_key,
         params.agent_types,
+        &state.emitter,
     )
     .await?;
     Ok(Json(result))