Bug: Delivery ETAs don't account for weekends

## Bug Report

### Description

Estimated delivery dates are calculated without accounting for weekends. A shipment picked up on a Friday with a **3-day Ground** service will show a delivery ETA of Monday — but the system is currently adding 3 calendar days, showing Sunday as the ETA. Carriers don't deliver on Sundays (and some don't deliver on Saturdays either).

### Steps to reproduce

1. Look at any Ground service shipment picked up on a Thursday or Friday
2. Compare the `estimatedDelivery` date in the data to what it *should* be with business-days-only calculation
3. The current dates in `shipments.js` are hardcoded, but any new shipment creation utility would reproduce this

### Expected behavior

ETA calculation should skip weekends (Saturday and Sunday) when computing delivery dates for Ground and Standard services. Overnight/Priority services may still use calendar days.

### Acceptance criteria

- [ ] A `calculateETA(pickupDate, serviceDays, service)` utility function exists in `src/utils/formatters.js`
- [ ] The function skips Saturdays and Sundays for Ground/Standard service types
- [ ] Overnight and Priority services continue to use calendar days
- [ ] Unit tests or inline examples in a comment demonstrate the correct behavior

---
*Demo app is in the `/demo` directory. Run `npm install && npm run dev` to start.*

Start by reading src/utils/formatters.js and inspect the hardcoded dates in shipments.js to understand the current ETA behavior. Run npm install && npm run dev in /demo if visual verification helps, then add the calculateETA utility and tests or inline examples covering weekend-skipping Ground/Standard services and calendar-day Overnight/Priority services.

Written by the indexing model from the issue text.