Phase 1: Infrastructure setup - Install next-intl and create basic i18n structure

- Install next-intl package
- Create LanguageProvider hook with localStorage persistence
- Create IntlProvider component for next-intl integration
- Create Providers wrapper component
- Update layout.tsx to include providers and set default lang to 'es'
- Create initial translation files (common.json) for es, en, ca
- Fix pre-existing TypeScript errors in various pages

All tests passing, build successful.

frontend/locales/es/common.json

@@ -0,0 +1,15 @@
+{
+  "loading": "Cargando...",
+  "error": "Error",
+  "save": "Guardar",
+  "cancel": "Cancelar",
+  "back": "Atrás",
+  "continue": "Continuar",
+  "confirm": "Confirmar",
+  "delete": "Eliminar",
+  "edit": "Editar",
+  "close": "Cerrar",
+  "yes": "Sí",
+  "no": "No",
+  "retry": "Reintentar"
+}