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/page.svelte.test.ts

@@ -246,7 +246,7 @@ describe('admin/system page', () => {
 				return new Response(
 					JSON.stringify({
 						state: 'FAILED',
-						message: 'database error',
+						statusCode: 'IMPORT_FAILED_INTERNAL',
 						processed: 0,
 						startedAt: null
 					}),
@@ -262,7 +262,7 @@ describe('admin/system page', () => {
 		render(AdminSystemPage, { props: {} });
 
 		await vi.waitFor(() => {
-			expect(document.body.textContent).toContain('database error');
+			expect(document.body.textContent).toContain('Interner Fehler beim Import');
 		});
 	});