feat(web): 官网移动端适配

- 导航栏：≤768 隐藏横向链接，显示汉堡按钮；点击展开下拉菜单，含全部链接和 CTA
- 新增 ≤600 手机断点：footer 单列、grid 单列、container 内边距收窄、标题字号下调
- 首页：≤768 隐藏 Hero mock 预览（装饰性），≤600 CTA 按钮纵向排列
- features 页（库存/审批）：hamburger 导航、≤768 隐藏 composite 视觉、≤600 footer 单列
- 文档页（docs.css）：≤600 表格横向可滚
- 下载页：≤600 平台卡片单列排列

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/assets/style.css

@@ -128,6 +128,41 @@ button { font-family: inherit; cursor: pointer; }
 .nav-dropdown-divider { height: 1px; background: var(--border-subtle); margin: 4px 0; }
 .nav-dropdown .logout { color: var(--danger-600); }
 
+/* Hamburger button (hidden on desktop) */
+.nav-hamburger {
+  display: none; flex-direction: column; justify-content: center; gap: 5px;
+  width: 40px; height: 40px; padding: 8px;
+  background: none; border: none; cursor: pointer;
+  border-radius: var(--radius-md);
+}
+.nav-hamburger span {
+  display: block; height: 2px; width: 22px;
+  background: var(--gray-700); border-radius: 2px;
+  transition: transform 0.2s, opacity 0.2s;
+}
+
+/* Mobile menu (hidden by default) */
+.nav-mobile-menu {
+  display: none; flex-direction: column;
+  background: var(--gray-0);
+  border-top: 1px solid var(--border-subtle);
+  padding: 8px 0 16px;
+}
+.nav-mobile-menu.open { display: flex; }
+.nav-mobile-menu a {
+  padding: 12px 24px; font-size: var(--text-md);
+  color: var(--gray-700); font-weight: 500;
+  transition: background var(--duration-fast) var(--ease-standard);
+}
+.nav-mobile-menu a:hover { background: var(--gray-50); color: var(--brand-900); }
+.nav-mobile-cta {
+  display: flex; flex-direction: column; gap: 8px;
+  padding: 12px 24px 0;
+  border-top: 1px solid var(--border-subtle);
+  margin-top: 8px;
+}
+.nav-mobile-cta .btn { justify-content: center; }
+
 /* -- Footer -- */
 footer {
   background: var(--gray-25);
@@ -456,4 +491,17 @@ footer {
 }
 @media (max-width: 768px) {
   .grid-3, .grid-4, .grid-5 { grid-template-columns: repeat(2, 1fr); }
+  /* Show hamburger, hide desktop nav links */
+  .nav-hamburger { display: flex; }
+  .topnav-links, .topnav-cta { display: none; }
+}
+@media (max-width: 600px) {
+  .topnav-inner { padding: 0 16px; }
+  .footer-grid { grid-template-columns: 1fr; gap: 32px; }
+  .footer-bottom { flex-direction: column; gap: 8px; text-align: center; }
+  .grid-3, .grid-4, .grid-5 { grid-template-columns: 1fr; }
+  .container { padding: 0 16px; }
+  .section { padding-top: 48px; padding-bottom: 48px; }
+  h1, .h1 { font-size: clamp(1.75rem, 7vw, 2.5rem); }
+  h2, .h2 { font-size: clamp(1.375rem, 5vw, 2rem); }
 }