fix(i18n): add message keys for viewer, transcribe, and layout controls

Adds 7 Paraglide keys (viewer_previous_page, viewer_next_page,
viewer_zoom_out, viewer_zoom_in, transcribe_mark_for_training,
layout_menu_open, layout_menu_close) to de/en/es.json.

Adds messages.spec.ts to enforce key parity across all three locales.

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

frontend/src/lib/messages.spec.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import de from '../../messages/de.json';
+import en from '../../messages/en.json';
+import es from '../../messages/es.json';
+
+describe('message key parity', () => {
+	it('de, en, and es have identical key sets', () => {
+		const deKeys = Object.keys(de).sort();
+		const enKeys = Object.keys(en).sort();
+		const esKeys = Object.keys(es).sort();
+		expect(enKeys).toEqual(deKeys);
+		expect(esKeys).toEqual(deKeys);
+	});
+
+	it('viewer navigation keys are present in all locales', () => {
+		const requiredViewerKeys = [
+			'viewer_previous_page',
+			'viewer_next_page',
+			'viewer_zoom_out',
+			'viewer_zoom_in'
+		];
+		for (const key of requiredViewerKeys) {
+			expect(de, `missing key in de: ${key}`).toHaveProperty(key);
+			expect(en, `missing key in en: ${key}`).toHaveProperty(key);
+			expect(es, `missing key in es: ${key}`).toHaveProperty(key);
+		}
+	});
+
+	it('transcribe mark-for-training key is present in all locales', () => {
+		expect(de).toHaveProperty('transcribe_mark_for_training');
+		expect(en).toHaveProperty('transcribe_mark_for_training');
+		expect(es).toHaveProperty('transcribe_mark_for_training');
+	});
+
+	it('layout menu open/close keys are present in all locales', () => {
+		expect(de).toHaveProperty('layout_menu_open');
+		expect(de).toHaveProperty('layout_menu_close');
+		expect(en).toHaveProperty('layout_menu_open');
+		expect(en).toHaveProperty('layout_menu_close');
+		expect(es).toHaveProperty('layout_menu_open');
+		expect(es).toHaveProperty('layout_menu_close');
+	});
+});