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/pubspec.yaml

@@ -0,0 +1,24 @@
+name: sxchnge_theme
+description: SXCHNGE Material theme and bundled Onest typography for Flutter apps.
+version: 0.1.0
+publish_to: none
+
+environment:
+  sdk: '>=3.13.0 <4.0.0'
+  flutter: '>=3.47.0'
+
+dependencies:
+  flutter:
+    sdk: flutter
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+
+flutter:
+  assets:
+    - assets/fonts/OFL.txt
+  fonts:
+    - family: Onest
+      fonts:
+        - asset: assets/fonts/Onest-Variable.ttf