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/test/integration_test.dart

@@ -0,0 +1,51 @@
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:sxchnge_theme_example/main.dart';
+
+void main() {
+  testWidgets('consumer bundles the package font and its license', (
+    tester,
+  ) async {
+    await tester.pumpWidget(const ThemeExample());
+    final manifest =
+        jsonDecode(await rootBundle.loadString('FontManifest.json')) as List;
+    final family = manifest.cast<Map>().singleWhere(
+      (font) => font['family'] == 'packages/sxchnge_theme/Onest',
+    );
+    final asset = (family['fonts'] as List).first['asset'] as String;
+    expect((await rootBundle.load(asset)).lengthInBytes, greaterThan(1000));
+    expect(
+      await rootBundle.loadString(
+        'packages/sxchnge_theme/assets/fonts/OFL.txt',
+      ),
+      contains('SIL OPEN FONT LICENSE'),
+    );
+  });
+
+  for (final mode in [ThemeMode.light, ThemeMode.dark]) {
+    testWidgets(
+      '$mode renders on a narrow screen with large text and accepts input',
+      (tester) async {
+        tester.view.physicalSize = const Size(320, 900);
+        tester.view.devicePixelRatio = 1;
+        tester.platformDispatcher.textScaleFactorTestValue = 2;
+        addTearDown(tester.view.resetPhysicalSize);
+        addTearDown(tester.view.resetDevicePixelRatio);
+        addTearDown(tester.platformDispatcher.clearTextScaleFactorTestValue);
+        await tester.pumpWidget(ThemeExample(mode: mode));
+        await tester.pumpAndSettle();
+        expect(tester.takeException(), isNull);
+        await tester.enterText(find.byType(TextField).first, '125.50');
+        expect(find.text('125.50'), findsOneWidget);
+        await tester.ensureVisible(find.text('Continue'));
+        await tester.pumpAndSettle();
+        expect(tester.takeException(), isNull);
+      },
+    );
+  }
+}