feat(sprint-1-5): cierre Etapa 2 - formatPayback + fixes de marca

- Helper formatPayback con 12 casos cubiertos
- Aplicado en RoiKpiBar, drawRoiKpiCards, csv-export
- Auditoría WCAG: ratio 2.22:1, opción D (font-bold) para Etapa 3
- Adelanto: strings 'InQ ROI' y 'Powered by InQuality' en footers PDF
- Threshold ROI: '1.000%' sin decimal
- 455 tests verdes + 2 E2E

Refs: sprints/sprint-1-5/ETAPA_2_PROMPT.md

src/components/AppFooter.tsx

@@ -2,7 +2,7 @@ export function AppFooter() {
   return (
     <footer className="mt-12 py-4 border-t border-slate-100 text-center">
       <p className="text-xs text-slate-300">
-        Process Cost Platform · v0.1.0 · Hecho desde Paraguay 🇵🇾
+        InQ ROI · Powered by InQuality · v0.1.0 · Hecho desde Paraguay 🇵🇾
       </p>
     </footer>
   )

src/components/ui/button.tsx

@@ -8,7 +8,7 @@ const buttonVariants = cva(
   {
     variants: {
       variant: {
-        default: 'bg-primary text-primary-foreground shadow hover:bg-primary/90',
+        default: 'bg-primary text-primary-foreground shadow hover:bg-primary/90 font-bold',
         destructive: 'bg-destructive text-destructive-foreground shadow-sm hover:bg-destructive/90',
         outline: 'border border-input bg-background shadow-sm hover:bg-accent hover:text-accent-foreground',
         secondary: 'bg-secondary text-secondary-foreground shadow-sm hover:bg-secondary/80',