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/packages/sxchnge_theme/lib/sxchnge_theme.dart

@@ -0,0 +1,5 @@
+/// SXCHNGE brand colors, typography, and Material themes.
+library;
+
+export 'src/brand.dart';
+export 'src/theme.dart';