Billing UI, redesigned

The billing dashboard was the last piece of our UI still running on the pre-2025 design system. It's now redesigned end to end.

New in this release:

• Balance forecasting — we predict when you'll run out based on your last 7d burn.

• Per-instance burn rate on the fleet view.

• CSV export for invoices (finally). Also PDF for the ones you need for accounting.

• Every snapshot line item now links back to the invoice it belongs to.