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

tests/hooks/useSettings.test.tsx

@@ -8,6 +8,7 @@ const useSettingsQueryMock = vi.fn();
 const setAppConfigDirOverrideMock = vi.fn();
 const applyClaudePluginConfigMock = vi.fn();
 const syncCurrentProvidersLiveMock = vi.fn();
+const updateTrayMenuMock = vi.fn();
 const toastErrorMock = vi.fn();
 const toastSuccessMock = vi.fn();
 
@@ -52,6 +53,9 @@ vi.mock("@/lib/api", () => ({
     syncCurrentProvidersLive: (...args: unknown[]) =>
       syncCurrentProvidersLiveMock(...args),
   },
+  providersApi: {
+    updateTrayMenu: (...args: unknown[]) => updateTrayMenuMock(...args),
+  },
 }));
 
 const createSettingsFormMock = (overrides: Record<string, unknown> = {}) => ({