refactor: 重构侧边栏布局，新增独立子菜单面板

Fixes: #36

test/sidebar-submenu-panel-and-theme-mode.node-test.js

@@ -0,0 +1,30 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+test('默认布局：应包含侧边栏分类面板容器（避免子菜单过长挤压页面列表）', () => {
+  const repoRoot = path.resolve(__dirname, '..');
+  const layoutPath = path.join(repoRoot, 'templates', 'layouts', 'default.hbs');
+  const content = fs.readFileSync(layoutPath, 'utf8');
+
+  assert.ok(
+    content.includes('sidebar-submenu-panel'),
+    'templates/layouts/default.hbs 应包含 sidebar-submenu-panel 容器'
+  );
+  assert.ok(
+    content.includes('data-container="sidebar-submenu"'),
+    'sidebar-submenu-panel 容器应包含 data-container="sidebar-submenu"'
+  );
+});
+
+test('默认布局：应输出 data-theme-mode，支持 dark/light/system 默认模式', () => {
+  const repoRoot = path.resolve(__dirname, '..');
+  const layoutPath = path.join(repoRoot, 'templates', 'layouts', 'default.hbs');
+  const content = fs.readFileSync(layoutPath, 'utf8');
+
+  assert.ok(
+    content.includes('data-theme-mode='),
+    'templates/layouts/default.hbs 应输出 data-theme-mode 属性'
+  );
+});