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/lib/main.dart

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+import 'package:flutter/material.dart';
+
+import 'package:sxchnge_theme/sxchnge_theme.dart';
+
+void main() => runApp(const ThemeExample());
+
+class ThemeExample extends StatelessWidget {
+  const ThemeExample({super.key, this.mode = ThemeMode.system});
+
+  final ThemeMode mode;
+
+  @override
+  Widget build(BuildContext context) => MaterialApp(
+    debugShowCheckedModeBanner: false,
+    theme: SxchngeTheme.light(),
+    darkTheme: SxchngeTheme.dark(),
+    themeMode: mode,
+    home: const _Gallery(),
+  );
+}
+
+class _Gallery extends StatelessWidget {
+  const _Gallery();
+
+  @override
+  Widget build(BuildContext context) => Scaffold(
+    appBar: AppBar(title: const Text('sxchnge')),
+    body: SingleChildScrollView(
+      padding: const EdgeInsets.all(24),
+      child: Center(
+        child: ConstrainedBox(
+          constraints: const BoxConstraints(maxWidth: 560),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.stretch,
+            children: [
+              Text(
+                'Your next exchange',
+                style: Theme.of(context).textTheme.headlineLarge,
+              ),
+              const SizedBox(height: 16),
+              const Text(
+                'Обмен без лишних шагов.\nA preview of the SXCHNGE theme.',
+              ),
+              const SizedBox(height: 24),
+              const Card(
+                child: Padding(
+                  padding: EdgeInsets.all(24),
+                  child: Text('Clear amounts. Clear choices.'),
+                ),
+              ),
+              const SizedBox(height: 24),
+              const TextField(
+                decoration: InputDecoration(
+                  labelText: 'Amount',
+                  hintText: '0.00',
+                ),
+              ),
+              const SizedBox(height: 16),
+              const TextField(
+                decoration: InputDecoration(
+                  labelText: 'Email',
+                  errorText: 'Enter a valid email address',
+                ),
+              ),
+              const SizedBox(height: 24),
+              FilledButton(onPressed: () {}, child: const Text('Continue')),
+              const SizedBox(height: 12),
+              OutlinedButton(onPressed: () {}, child: const Text('Back')),
+              const SizedBox(height: 12),
+              const FilledButton(onPressed: null, child: Text('Unavailable')),
+            ],
+          ),
+        ),
+      ),
+    ),
+  );
+}

frontend/packages/sxchnge_theme/example/pubspec.yaml

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+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

frontend/packages/sxchnge_theme/example/test/integration_test.dart

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+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);
+      },
+    );
+  }
+}