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

@@ -0,0 +1,77 @@
+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')),
+            ],
+          ),
+        ),
+      ),
+    ),
+  );
+}