Add cálculo de trial y badge de plan

mvp/b2c/tests/billing/plan.test.ts

@@ -0,0 +1,21 @@
+import { describe, it, expect } from 'vitest';
+import { trialDaysRemaining, formatPlanBadge } from '@/lib/billing/plan';
+
+describe('plan', () => {
+  it('calcula días de trial restantes (redondeo hacia arriba, nunca negativo)', () => {
+    const now = new Date('2026-05-30T12:00:00Z');
+    expect(trialDaysRemaining(new Date('2026-06-05T12:00:00Z'), now)).toBe(6);
+    expect(trialDaysRemaining(new Date('2026-05-30T18:00:00Z'), now)).toBe(1);
+    expect(trialDaysRemaining(new Date('2026-05-29T12:00:00Z'), now)).toBe(0);
+    expect(trialDaysRemaining(null, now)).toBe(0);
+  });
+
+  it('formatea el badge según estado', () => {
+    const now = new Date('2026-05-30T12:00:00Z');
+    expect(formatPlanBadge('Pro', 'trial', new Date('2026-06-05T12:00:00Z'), now))
+      .toBe('Plan Pro · trial, 6 días restantes');
+    expect(formatPlanBadge('Pro', 'activo', null, now)).toBe('Plan Pro · activo');
+    expect(formatPlanBadge(null, 'trial', new Date('2026-06-05T12:00:00Z'), now))
+      .toBe('Sin plan · trial, 6 días restantes');
+  });
+});