feat(client): context.tokens 取色扩展

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

client/lib/core/theme/context_tokens.dart

@@ -0,0 +1,6 @@
+import 'package:flutter/material.dart';
+import 'app_tokens.dart';
+
+extension TokensX on BuildContext {
+  AppTokens get tokens => Theme.of(this).extension<AppTokens>()!;
+}

client/test/theme/context_tokens_test.dart

@@ -0,0 +1,15 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:jiu_client/core/theme/context_tokens.dart';
+import 'package:jiu_client/core/theme/themes.dart';
+
+void main() {
+  testWidgets('context.tokens 取到当前主题色', (tester) async {
+    late Color seen;
+    await tester.pumpWidget(MaterialApp(
+      theme: buildTheme('a'),
+      home: Builder(builder: (ctx) { seen = ctx.tokens.primary; return const SizedBox(); }),
+    ));
+    expect(seen, const Color(0xFF2563AC));
+  });
+}