Feature: Add CSV export for shipment history

## Feature Request

### Description

Operations teams need to export shipment data for weekly reporting. Add a **"Export CSV"** button to the shipment dashboard that downloads the currently filtered/visible shipments as a CSV file.

### Requirements

- Button should appear in the table toolbar (top-right area)
- Export should respect the current search filter and status filter — only export what's visible
- CSV columns: Tracking #, Customer, Reference, Origin, Destination, Status, Carrier, Service, Weight, Est. Delivery
- Filename format: `belcher-freight-export-YYYY-MM-DD.csv`

### Acceptance criteria

- [ ] "Export CSV" button visible in table toolbar
- [ ] Clicking it downloads a `.csv` file
- [ ] File contains only currently-visible rows (after search/filter)
- [ ] Column headers match the spec above
- [ ] Filename includes today's date
- [ ] Works in Chrome, Firefox, and Safari

### Notes

No backend required — use the [Blob API](https://developer.mozilla.org/en-US/docs/Web/API/Blob) to generate and download the file client-side.

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

Start in the /demo directory with npm install && npm run dev, then locate the shipment dashboard and its table toolbar. Use the existing visible, filtered shipment rows and the Blob API as described. Done means the toolbar button downloads the specified columns, filtered rows, and dated filename in Chrome, Firefox, and Safari.

Written by the indexing model from the issue text.