feat(settings): add model provider management with full CRUD support

Add a new settings page for managing API model providers (name, API URL,
API key, applicable agent types). Includes database migration, SeaORM
entity, backend CRUD commands/handlers, frontend settings UI with agent
type filter, add/edit/delete dialogs, and i18n support for all 10 locales.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/mod.rs

@@ -4,6 +4,7 @@ pub mod conversations;
 pub mod folder_commands;
 pub mod folders;
 pub mod mcp;
+pub mod model_provider;
 pub mod project_boot;
 pub mod system_settings;
 pub mod terminal;