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/integration/App.test.tsx

@@ -109,8 +109,8 @@ vi.mock("@/components/ConfirmDialog", () => ({
     ) : null,
 }));
 
-vi.mock("@/components/settings/SettingsDialog", () => ({
-  SettingsDialog: ({ open, onOpenChange, onImportSuccess }: any) =>
+vi.mock("@/components/settings/SettingsPage", () => ({
+  SettingsPage: ({ open, onOpenChange, onImportSuccess }: any) =>
     open ? (
       <div data-testid="settings-dialog">
         <button onClick={() => onImportSuccess?.()}>