feat(client): 三套 ThemeData 构建（挂 AppTokens，B 为 dark）

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/core/theme/themes.dart

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+import 'package:flutter/material.dart';
+import 'app_tokens.dart';
+import 'app_tokens.g.dart';
+
+ThemeData buildTheme(String key) {
+  final t = appTokensOf(key);
+  final brightness = t.isDark ? Brightness.dark : Brightness.light;
+  final scheme = ColorScheme.fromSeed(
+    seedColor: t.primary,
+    brightness: brightness,
+  ).copyWith(
+    primary: t.primary,
+    onPrimary: t.onPrimary,
+    surface: t.surface,
+    error: t.danger,
+  );
+  return ThemeData(
+    useMaterial3: true,
+    brightness: brightness,
+    colorScheme: scheme,
+    scaffoldBackgroundColor: t.bg,
+    fontFamily: null, // 系统中文字体；字号梯度后续在组件层接入
+    extensions: <ThemeExtension<dynamic>>[t],
+  );
+}

client/test/theme/themes_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:jiu_client/core/theme/app_tokens.dart';
+import 'package:jiu_client/core/theme/themes.dart';
+
+void main() {
+  test('buildTheme 挂载 AppTokens 且亮暗正确', () {
+    final a = buildTheme('a');
+    expect(a.extension<AppTokens>(), isNotNull);
+    expect(a.brightness, Brightness.light);
+    expect(a.extension<AppTokens>()!.primary, const Color(0xFF2563AC));
+
+    final b = buildTheme('b');
+    expect(b.brightness, Brightness.dark);
+    expect(b.scaffoldBackgroundColor, b.extension<AppTokens>()!.bg);
+  });
+}