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.

frontend/README.md

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+# SXCHNGE frontend
+
+Home for the consumer application and its Flutter packages. The existing public Astro website lives in [`../website/`](../website/README.md).
+
+The first package, [`sxchnge_theme`](packages/sxchnge_theme/README.md), provides installable light/dark Material themes and bundled Onest fonts. Its `example/` folder demonstrates installation and component styling. The consumer application in [`app/`](app/README.md) now includes a responsive login/sign-up preview; backend authentication is not connected.
+
+The application will provide consumer registration, sign-in, onboarding and verification screens, exchange flows, and transaction history through the sxchnge backend. Eligibility and remaining limits are projections of Sendico's authoritative decisions.
+
+Flutter is anticipated by the existing website handoff. Optional brand references are in [`../website/design/`](../website/design/README.md) and [`../website/flutter/brand_palette.dart`](../website/flutter/brand_palette.dart); these do not prescribe the application's component styling.
+
+Define the first customer journey and BFF contract before scaffolding the application. Sendico integration credentials belong in the backend.
+
+Shared authentication: [`sxchnge_auth`](packages/sxchnge_auth/README.md) provides reusable login/signup providers, service contracts, and dummy implementations for both web and future mobile clients.