fix: pre-release fixes for code formatting, i18n, and test suite

- Run cargo fmt to fix Rust code formatting (lib.rs)
- Add missing i18n keys: migration.success, agents.title (zh/en/ja)
- Replace hardcoded strings "Agents" and "MCP" with t() calls in App.tsx
- Fix test mocks and assertions:
  - Add providersApi.updateTrayMenu to useSettings.test.tsx mock
  - Update SettingsPage mock path in App.test.tsx
  - Fix toast message assertion in integration/SettingsDialog.test.tsx
  - Add autoSaveSettings to SettingsDialog component test mock
  - Fix loading state test to check spinner instead of title
  - Update import button name matching for selected file state
  - Fix save button test to switch to advanced tab first
  - Remove obsolete cancel button test (button no longer exists)

Test results improved from 99 passed / 17 failed to 104 passed / 11 failed

src-tauri/src/lib.rs

@@ -71,7 +71,8 @@ impl TrayTexts {
             },
             "ja" => Self {
                 show_main: "メインウィンドウを開く",
-                no_provider_hint: "  (プロバイダーがまだありません。メイン画面から追加してください)",
+                no_provider_hint:
+                    "  (プロバイダーがまだありません。メイン画面から追加してください)",
                 quit: "終了",
             },
             _ => Self {
@@ -521,7 +522,8 @@ pub fn run() {
 
                                 unsafe {
                                     use objc2::msg_send;
-                                    let _: () = msg_send![&*ns_window, setBackgroundColor: &*bg_color];
+                                    let _: () =
+                                        msg_send![&*ns_window, setBackgroundColor: &*bg_color];
                                 }
                             } else {
                                 log::warn!("Failed to retain NSWindow reference");