feat(render): 布局模板画廊 —— live 缩略预览 + 点选完整预览

- LayoutGallery 升级:每模板卡片 = 真实渲染缩略图(scale 0.4),点选看完整预览
- demoPageFrom:由 preview.demoBlocks 构造可渲染 demo 页(预览即真实)
- playground 新增 dashboard 布局 + stat-grid widget;app-shell 验证 DefaultLayout 兜底
- 三个模板(admin/dashboard/app-shell)全部 live 渲染验证通过

examples/playground/src/widgets.tsx

@@ -70,3 +70,33 @@ export function NavMenu({ items = [] }: { items?: Array<{ label: string; active?
     ),
   )
 }
+
+/** 统计卡片(仪表盘用) */
+export function StatCard({
+  label,
+  value,
+  delta,
+}: {
+  label?: string
+  value?: unknown
+  delta?: string
+}): React.ReactElement {
+  return React.createElement(
+    'div',
+    { style: { border: '1px solid #e5e7eb', borderRadius: 8, padding: 16, background: '#fff' } },
+    React.createElement('div', { style: { fontSize: 12, color: '#6b7280' } }, label ?? ''),
+    React.createElement('div', { style: { fontSize: 24, fontWeight: 700, margin: '4px 0' } }, String(value ?? '-')),
+    delta ? React.createElement('div', { style: { fontSize: 12, color: delta.startsWith('-') ? '#dc2626' : '#16a34a' } }, delta) : null,
+  )
+}
+
+/** 指标格子容器(dashboard 布局的内容区 demo 用) */
+export function StatGrid({ stats = [] }: { stats?: Array<{ label?: string; value?: unknown; delta?: string }> }): React.ReactElement {
+  return React.createElement(
+    'div',
+    { style: { display: 'grid', gridTemplateColumns: 'repeat(auto-fit, minmax(160px, 1fr))', gap: 12 } },
+    ...(stats as Array<{ label?: string; value?: unknown; delta?: string }>).map((s, i) =>
+      React.createElement(StatCard, { key: i, label: s.label, value: s.value, delta: s.delta }),
+    ),
+  )
+}