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/db/migration/mod.rs

@@ -7,6 +7,7 @@ mod m20260226_000001_agent_setting;
 mod m20260227_000001_folder_parent_branch;
 mod m20260330_000001_chat_channel;
 mod m20260401_000001_chat_channel_sender_context;
+mod m20260404_000001_model_provider;
 pub struct Migrator;
 
 #[async_trait::async_trait]
@@ -20,6 +21,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260227_000001_folder_parent_branch::Migration),
             Box::new(m20260330_000001_chat_channel::Migration),
             Box::new(m20260401_000001_chat_channel_sender_context::Migration),
+            Box::new(m20260404_000001_model_provider::Migration),
         ]
     }
 }