feat(frontend): scaffold i18n init, fr.json (~140 keys), status map, format helpers

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

frontend/tests/i18n.test.ts

@@ -0,0 +1,46 @@
+import { describe, expect, it } from 'vitest';
+import i18n from '@/i18n/index';
+import { engagementStatusLabel, simulationStatusLabel } from '@/i18n/status';
+import { formatDate, formatDateTime } from '@/lib/format';
+
+describe('i18n smoke tests', () => {
+  it('resolves common.save to French', () => {
+    expect(i18n.t('common.save')).toBe('Enregistrer');
+  });
+
+  it('missing key returns the key path', () => {
+    expect(i18n.t('this.key.does.not.exist')).toBe('this.key.does.not.exist');
+  });
+
+  it('simulationStatusLabel done → Terminé', () => {
+    expect(simulationStatusLabel('done')).toBe('Terminé');
+  });
+
+  it('engagementStatusLabel active → Actif', () => {
+    expect(engagementStatusLabel('active')).toBe('Actif');
+  });
+
+  it('interpolation inserts the value', () => {
+    expect(i18n.t('engagement.list.deleteConfirm', { name: 'Test' })).toContain('Test');
+  });
+});
+
+describe('format helpers', () => {
+  it('formatDate returns — for null', () => {
+    expect(formatDate(null)).toBe('—');
+  });
+
+  it('formatDate returns — for undefined', () => {
+    expect(formatDate(undefined)).toBe('—');
+  });
+
+  it('formatDate formats ISO date in fr-FR', () => {
+    const result = formatDate('2026-06-01');
+    expect(result).toMatch(/\d{2}\/\d{2}\/\d{4}/);
+  });
+
+  it('formatDateTime returns a non-empty string for valid ISO', () => {
+    const result = formatDateTime('2026-06-01T08:00:00');
+    expect(result.length).toBeGreaterThan(0);
+  });
+});