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/test/theme_test.dart

@@ -0,0 +1,65 @@
+import 'dart:convert';
+import 'dart:io';
+import 'dart:math' as math;
+
+import 'package:flutter/material.dart';
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:sxchnge_theme/sxchnge_theme.dart';
+
+double contrast(Color a, Color b) {
+  final x = a.computeLuminance();
+  final y = b.computeLuminance();
+  return (math.max(x, y) + 0.05) / (math.min(x, y) + 0.05);
+}
+
+void main() {
+  test('website core colors stay aligned with the package snapshot', () {
+    final source = File('../../../website/design/tokens.json');
+    if (!source.existsSync())
+      return; // Also allow testing outside this monorepo.
+    final colors =
+        (jsonDecode(source.readAsStringSync()) as Map)['colors'] as Map;
+    for (final entry in {
+      'ink': SxchngeBrand.ink,
+      'mint': SxchngeBrand.mint,
+      'lime': SxchngeBrand.lime,
+      'paper': SxchngeBrand.paper,
+      'forest': SxchngeBrand.forest,
+    }.entries) {
+      expect(
+        entry.value.toARGB32(),
+        int.parse('ff${(colors[entry.key] as String).substring(1)}', radix: 16),
+      );
+    }
+  });
+
+  for (final dark in [false, true]) {
+    final theme = dark ? SxchngeTheme.dark() : SxchngeTheme.light();
+    test(
+      '${dark ? "dark" : "light"} foreground pairs meet normal text contrast',
+      () {
+        final c = theme.colorScheme;
+        for (final pair in [
+          (c.primary, c.onPrimary),
+          (c.primaryContainer, c.onPrimaryContainer),
+          (c.secondary, c.onSecondary),
+          (c.secondaryContainer, c.onSecondaryContainer),
+          (c.tertiary, c.onTertiary),
+          (c.tertiaryContainer, c.onTertiaryContainer),
+          (c.surface, c.onSurface),
+          (c.surface, c.onSurfaceVariant),
+          (c.error, c.onError),
+          (c.errorContainer, c.onErrorContainer),
+        ]) {
+          expect(contrast(pair.$1, pair.$2), greaterThanOrEqualTo(4.5));
+        }
+        expect(
+          theme.textTheme.bodyMedium!.fontFamily,
+          'packages/sxchnge_theme/Onest',
+        );
+      },
+    );
+  }
+}