Prepare product layout and reusable Flutter theme

Relocate the existing Astro website and preserve its assets and hosting configuration. Add the consumer/backend scaffold and themed Flutter package with consumer tests. Related to #1.

backend/README.md

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+# SXCHNGE backend
+
+Reserved for a small backend for frontend (BFF) using a supported Sendico API. No service, runtime, or dependencies have been initialized yet.
+
+Planned responsibilities:
+
+- Consumer authentication and sessions, with a stable mapping to organization-scoped Sendico customer references.
+- Consumer-authorized access to onboarding, quotes, orders, and transaction history.
+- Product-specific orchestration and presentation of Sendico KYC, eligibility, and remaining-limit responses.
+- Server-side integration credentials and request correlation with Sendico.
+
+Sendico owns payment execution, authoritative KYC evidence and verification records, policy evaluation, and limit reservations/enforcement. Do not copy its existing BFF or import local Sendico implementation modules.
+
+Before implementation, agree on customer registration and identity mapping, customer KYC operations, eligibility/limits reads, customer-scoped payment access, and retry/idempotency behavior. Validate consumer isolation even when multiple customers use the same sxchnge organization and integration principal.