Costs, usage, invoices, and payments now use real data

The costs, usage, invoice detail, and payment detail pages were previously showing placeholder or invented data. They now read from and write to the live APIs, so every number and action reflects your actual account.

Four dashboard pages that previously displayed stub data or silently faked success have been wired to the live APIs.

**Costs page** now pulls from the real cost APIs and no longer shows invented KPIs.

**Usage page** now reads from the live metering APIs, so usage figures match what is actually recorded.

**Invoice detail page** - the "Send invoice" action now submits to the real endpoint and persists the result.

**Payment detail page** - the "Refund" action now calls the real refund endpoint; refunds are processed and recorded instead of returning a fake success message.