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/example/pubspec.yaml

@@ -0,0 +1,15 @@
+name: sxchnge_theme_example
+description: Minimal consumer and component gallery for the SXCHNGE theme.
+publish_to: none
+environment:
+  sdk: '>=3.13.0 <4.0.0'
+dependencies:
+  flutter:
+    sdk: flutter
+  sxchnge_theme:
+    path: ../
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+flutter:
+  uses-material-design: true