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.spec.ts

@@ -163,7 +163,7 @@ describe('Admin system page — mass import card', () => {
 					ok: true,
 					json: async () => ({
 						state: 'FAILED',
-						message: 'Datei nicht gefunden.',
+						statusCode: 'IMPORT_FAILED_NO_SPREADSHEET',
 						processed: 0,
 						startedAt: '2026-01-01T10:00:00'
 					})
@@ -182,7 +182,7 @@ describe('Admin system page — mass import card', () => {
 				})
 		);
 		render(Page, {});
-		await expect.element(page.getByText(/Datei nicht gefunden/i)).toBeInTheDocument();
+		await expect.element(page.getByText(/Keine Tabellendatei gefunden/i)).toBeInTheDocument();
 		await expect.element(page.getByRole('button', { name: /Erneut starten/i })).toBeInTheDocument();
 	});
 });