chore: 全站改为免费+联盟模式，原支付保留备用

- 课程详情页下架付费购买与扫码支付弹窗
- 首页/能力导览文案 购买→免费使用
- 付费面(会员/技能广场/用量包/课程)均已免费化，原 orders/payment 模块保留后端备用

frontend/src/app/courses/[id]/client.tsx

@@ -5,7 +5,7 @@ import { useParams } from 'next/navigation';
 import Link from 'next/link';
 import { Skeleton } from '@/components/ui/skeleton';
 import { API_BASE } from '@/lib/config';
-import { BookOpen, AlertTriangle, Menu, ChevronLeft, X } from 'lucide-react';
+import { BookOpen, AlertTriangle, Menu, ChevronLeft } from 'lucide-react';
 
 interface Lesson {
   id: number; title: string; content?: string; sortOrder: number; status: string;
@@ -33,39 +33,6 @@ function LoadingSkeleton() {
   );
 }
 
-function PayModal({ paying, qrCodeUrl, onClose }: { paying: boolean; qrCodeUrl: string; onClose: () => void }) {
-  if (!paying) return null;
-  return (
-    <div className="fixed inset-0 bg-black/50 z-50 flex items-center justify-center p-4">
-      <div className="bg-card rounded-2xl p-8 max-w-md w-full text-center shadow-xl border border-border">
-        <div className="flex items-center justify-between mb-4">
-          <h3 className="text-lg font-semibold text-foreground">扫码支付</h3>
-          <button onClick={onClose} className="p-1 rounded-lg hover:bg-muted">
-            <X className="w-5 h-5 text-muted-foreground" />
-          </button>
-        </div>
-        {qrCodeUrl ? (
-          <>
-            <div className="bg-muted/50 rounded-xl p-6 mb-4 inline-block">
-              <img
-                src={`https://api.qrserver.com/v1/create-qr-code/?size=200x200&data=${encodeURIComponent(qrCodeUrl)}`}
-                alt="支付二维码"
-                className="rounded-lg"
-              />
-            </div>
-            <p className="text-sm text-muted-foreground mb-4">请使用微信扫描二维码完成支付</p>
-          </>
-        ) : (
-          <div className="py-12">
-            <div className="w-10 h-10 border-2 border-brand-600 border-t-transparent rounded-full animate-spin mx-auto mb-4" />
-            <p className="text-sm text-muted-foreground">正在生成支付二维码...</p>
-          </div>
-        )}
-      </div>
-    </div>
-  );
-}
-
 export default function CourseDetailClient() {
   const params = useParams();
   const [course, setCourse] = useState<Course | null>(null);
@@ -73,8 +40,6 @@ export default function CourseDetailClient() {
   const [error, setError] = useState('');
   const [activeLesson, setActiveLesson] = useState<Lesson | null>(null);
   const [sidebarOpen, setSidebarOpen] = useState(true);
-  const [paying, setPaying] = useState(false);
-  const [qrCodeUrl, setQrCodeUrl] = useState('');
 
   useEffect(() => {
     if (!params.id) return;
@@ -107,11 +72,6 @@ export default function CourseDetailClient() {
 
   return (
     <>
-      <PayModal
-        paying={paying}
-        qrCodeUrl={qrCodeUrl}
-        onClose={() => { setPaying(false); setQrCodeUrl(''); }}
-      />
 
       <div className="flex h-[calc(100vh-4rem)] max-w-7xl mx-auto px-4 sm:px-6 lg:px-8">
         {/* Sidebar */}
@@ -185,41 +145,9 @@ export default function CourseDetailClient() {
             </div>
 
             <div className="flex items-center gap-2 text-xs text-muted-foreground shrink-0">
-              {course.isFree ? (
-                <span className="bg-green-100 dark:bg-green-950/30 text-green-700 dark:text-green-400 px-2 py-0.5 rounded font-medium">
-                  免费
-                </span>
-              ) : (
-                <>
-                  <span className="bg-brand-100 dark:bg-brand-950/30 text-brand-700 dark:text-brand-400 px-2 py-0.5 rounded font-medium">
-                    ¥{course.price}
-                  </span>
-                  <button
-                    onClick={async () => {
-                      setPaying(true);
-                      try {
-                        const res = await fetch(`${API_BASE}/payment/wxpay/unified-order`, {
-                          method: 'POST',
-                          headers: { 'Content-Type': 'application/json' },
-                          body: JSON.stringify({
-                            description: course.title,
-                            outTradeNo: `COURSE_${course.id}_${Date.now()}`,
-                            amount: course.price,
-                            tradeType: 'NATIVE',
-                          }),
-                        });
-                        const data = await res.json();
-                        if (data.codeUrl) {
-                          setQrCodeUrl(data.codeUrl);
-                        }
-                      } catch {}
-                    }}
-                    className="px-3 py-1 bg-brand-600 text-white text-xs rounded-lg hover:bg-brand-700 transition-colors font-medium"
-                  >
-                    立即购买
-                  </button>
-                </>
-              )}
+              <span className="bg-green-100 dark:bg-green-950/30 text-green-700 dark:text-green-400 px-2 py-0.5 rounded font-medium">
+                免费
+              </span>
               {course.category && (
                 <span className="bg-muted px-2 py-0.5 rounded">{course.category.name}</span>
               )}

frontend/src/app/page.tsx

@@ -234,7 +234,7 @@ export default function HomePage() {
               { icon: BarChart3, title: '数据分析', desc: '分析数据、生成图表、洞察趋势，让数据说话', href: '/sandbox', cta: '打开沙盒' },
               { icon: FileText, title: '提示词工程', desc: '学习编写高效提示词，掌握与 AI 沟通的最佳实践', href: '/prompts', cta: '浏览提示词库' },
               { icon: GraduationCap, title: '技能实战', desc: '在真实场景中练习 AI 技能，获得即时评分反馈', href: '/practices', cta: '开始练习' },
-              { icon: Puzzle, title: '工作流技能', desc: '购买即用的 AI 工作流技能，解决具体业务问题', href: '/marketplace', cta: '前往技能广场' },
+              { icon: Puzzle, title: '工作流技能', desc: '免费使用的 AI 工作流技能，解决具体业务问题', href: '/marketplace', cta: '前往技能广场' },
             ].map((item) => {
               const Icon = item.icon;
               return (