feat: three nuxt frontends, demo seed, rounding + money-exponent + rate-cast fixes, archived specs

openspec/specs/invoice/spec.md

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+# invoice Specification
+
+## Purpose
+TBD - created by archiving change fulfillment-invoices. Update Purpose after archive.
+## Requirements
+### Requirement: Invoice request
+Customers SHALL request at most one open invoice (发票) per order, with title, kind (`personal | company`), and tax_no required for company invoices. Amount equals the order total in the order currency.
+
+#### Scenario: company invoice requires tax number
+- **WHEN** requesting a company invoice without tax_no
+- **THEN** the API returns 400
+
+#### Scenario: duplicate rejected
+- **WHEN** an order already has a requested or issued invoice
+- **THEN** a second request returns 409
+
+### Requirement: Invoice issuance
+Merchants SHALL issue requested invoices of their own shop's orders; issuing sets invoice_no, issued_at and status `issued`.
+
+#### Scenario: issue flow
+- **WHEN** the shop issues a requested invoice
+- **THEN** the customer sees status `issued` with an invoice number
+