fix: add missing imports to test files

Author: AlexMikhalev

desktop/src/lib/BackButton.test.ts

@@ -1,5 +1,6 @@
 import { fireEvent, render, screen } from '@testing-library/svelte';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { Button } from 'svelma';
 import BackButton from './BackButton.svelte';
 
 // Mock window.history and window.location

desktop/src/lib/Chat/__tests__/ContextEditModal.test.ts

@@ -1,6 +1,7 @@
 import { fireEvent, render, screen, waitFor } from '@testing-library/svelte';
 import { afterEach, beforeEach, describe, expect, it } from 'vitest';
 import '@testing-library/jest-dom';
+import { Modal } from 'svelma';
 
 import { createTestConfig } from '../../../__test-utils__/testConfig';
 // Test utilities for real API testing

desktop/src/lib/StartupScreen.test.ts

@@ -1,5 +1,6 @@
 import { render, screen } from '@testing-library/svelte';
 import { describe, expect, it, vi } from 'vitest';
+import { theme } from '../stores';
 import StartupScreen from './StartupScreen.svelte';
 
 // Mock Tauri APIs