fix(test): update import-status test mocks and imports for statusCode-based i18n

Three test files were written against the old API shape (raw `message` field) before
the statusCode i18n field was introduced, or used the wrong `expect` import path:

- ImportStatusCard.svelte.test.ts: `@vitest/browser/context` does not export `expect`
  in this project's Vitest setup — use `vitest` like every other test file.
- page.svelte.spec.ts: FAILED mock lacked `statusCode`; assertion matched old German
  raw message instead of the i18n string for IMPORT_FAILED_NO_SPREADSHEET.
- page.svelte.test.ts: same pattern — mock lacked `statusCode`; assertion checked for
  raw backend string "database error" instead of the rendered i18n text.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/system/ImportStatusCard.svelte.test.ts

@@ -1,6 +1,5 @@
-import { describe, it, vi } from 'vitest';
+import { describe, expect, it, vi } from 'vitest';
 import { render } from 'vitest-browser-svelte';
-import { expect } from '@vitest/browser/context';
 import ImportStatusCard from './ImportStatusCard.svelte';
 
 type ImportStatus = {