fix(search): replace languageTag() with getLocale(); sync KI→Smart in tests

Paraglide 2.5 runtime exports getLocale(), not languageTag(). The
8bed0cc6 commit introduced the wrong import when threading lang through
the NL search path.

Also updates two test assertions that still expected the old 'KI' button
label after 0b31a51e renamed it to 'Smart-Suche'.

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

frontend/src/routes/SearchFilterBar.svelte.spec.ts

@@ -209,7 +209,7 @@ describe('SearchFilterBar – smart-mode chip lifecycle hooks', () => {
 			smartMode: true,
 			onModeToggle
 		});
-		await page.getByRole('button', { name: /KI/ }).click();
+		await page.getByRole('button', { name: /Smart/ }).click();
 		expect(onModeToggle).toHaveBeenCalledOnce();
 	});
 

frontend/src/routes/documents/+page.svelte

@@ -17,7 +17,7 @@ import { bulkSelectionStore } from '$lib/document/bulkSelection.svelte';
 import { getErrorMessage, parseBackendError } from '$lib/shared/errors';
 import { csrfFetch } from '$lib/shared/cookies';
 import * as m from '$lib/paraglide/messages.js';
-import { languageTag } from '$lib/paraglide/runtime';
+import { getLocale } from '$lib/paraglide/runtime';
 import type { components } from '$lib/generated/api';
 
 type NlQueryInterpretation = components['schemas']['NlQueryInterpretation'];
@@ -225,7 +225,7 @@ async function runSmartSearch() {
 		const res = await csrfFetch('/api/search/nl', {
 			method: 'POST',
 			headers: { 'Content-Type': 'application/json' },
-			body: JSON.stringify({ query, lang: languageTag() })
+			body: JSON.stringify({ query, lang: getLocale() })
 		});
 		if (!res.ok) {
 			const backend = await parseBackendError(res);

frontend/src/routes/search/SmartModeToggle.svelte.spec.ts

@@ -22,7 +22,7 @@ describe('SmartModeToggle', () => {
 	it('shows the smart label when smartMode is true', async () => {
 		render(SmartModeToggle, { smartMode: true });
 		const btn = page.getByRole('button');
-		await expect.element(btn).toHaveTextContent('KI');
+		await expect.element(btn).toHaveTextContent('Smart');
 	});
 
 	it('shows the keyword label when smartMode is false', async () => {