feat(last-mile): customer-signed LM contract gates the advance invoice

Approval now snapshots the rate estimate and generates a last-mile
contract instead of invoicing immediately. The customer picks a delivery
date on the confirm form, then reviews and signs the contract in the
portal (saved signature or drawn); the signed PDF is stored as
LM_<CustomerName>.pdf and only then is the advance invoice issued.
Backoffice shows signature status and the contract download.

apps/edr-freight-web/backoffice/src/constants/URLS.ts

@@ -714,6 +714,7 @@ export const URL_CONSTANTS = {
     PRICE_ESTIMATE: (id: string) => `/last-mile-requests/${id}/price-estimate`,
     APPROVE: (id: string) => `/last-mile-requests/${id}/approve`,
     REJECT: (id: string) => `/last-mile-requests/${id}/reject`,
+    CONTRACT_DOCUMENT: (id: string) => `/last-mile-requests/${id}/contract/document`,
   },
 
   DRIVERS: {