Feature: Add shipment tracking timeline to detail panel

## Feature Request

### Description

The shipment detail panel currently shows tracking events as a basic list. Improve this to a **visual timeline** with progress indicators, making it much easier to understand where a shipment is in its journey at a glance.

### Design spec

- Show a vertical stepper/timeline with one step per event
- Each step has: timestamp, location, status description
- The **most recent event** should be visually highlighted (larger dot, bolder text)
- Completed steps (all except the last) should appear in a muted/gray style
- Add a **progress bar** at the top of the detail panel showing overall completion: Processing → In Transit → Out for Delivery → Delivered
- For delayed shipments, show a warning indicator ⚠️ on the affected step

### Acceptance criteria

- [ ] Timeline replaces the current flat event list in `ShipmentDetail.jsx`
- [ ] Progress bar shows the current stage in the delivery lifecycle
- [ ] Delayed status shows a visual warning on the relevant timeline step
- [ ] Design is consistent with the existing color scheme
- [ ] Works for shipments with 1 event and shipments with 5+ events

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

Run `npm install && npm run dev` in `/demo`, then inspect `ShipmentDetail.jsx` and the current tracking-event list. Replace that list with the specified timeline and lifecycle progress bar, including recent-event highlighting, delayed warnings, consistent colors, and support for one or many events. Verify each acceptance criterion in the running demo.

Written by the indexing model from the issue text.