fix: 修复外部资源、图标模式与嵌套交互（#30）

Fixes: https://github.com/rbetree/menav/issues/30

- Font Awesome：bootcdn→Cloudflare cdnjs
- favicon：gstatic `.com` 失败自动回退 `.cn`
- `icons.mode`：修复 `site.yml` 配置未生效（提升到顶层）
- 站点级图标覆盖：支持 `faviconUrl` / `forceIconMode`（优先级：`faviconUrl` > `forceIconMode` > `icons.mode`）
- 折叠交互：恢复二级分组折叠按钮（桌面端悬停显示）
- 新标签页：递归补齐多级 `sites.external` 默认值

test/multi-level-sites-external-default.node-test.js

@@ -0,0 +1,70 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+
+const { loadConfig } = require('../src/generator.js');
+
+function withTempCwd(callback) {
+  const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'menav-multilevel-external-test-'));
+  const originalCwd = process.cwd();
+
+  try {
+    process.chdir(tmpDir);
+    callback(tmpDir);
+  } finally {
+    process.chdir(originalCwd);
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  }
+}
+
+test('多级结构下 sites.external 未配置时应默认 true，且 external:false 不应被覆盖', () => {
+  withTempCwd((tmpDir) => {
+    const defaultConfigDir = path.join(tmpDir, 'config', '_default');
+    const defaultPagesDir = path.join(defaultConfigDir, 'pages');
+    fs.mkdirSync(defaultPagesDir, { recursive: true });
+
+    fs.writeFileSync(path.join(defaultConfigDir, 'site.yml'), 'title: Test\n', 'utf8');
+
+    fs.writeFileSync(
+      path.join(defaultPagesDir, 'bookmarks.yml'),
+      [
+        'title: 书签',
+        'subtitle: bookmarks',
+        'template: bookmarks',
+        'categories:',
+        '  - name: 技术资源',
+        '    groups:',
+        '      - name: 组内站点',
+        '        sites:',
+        '          - name: GroupSite',
+        '            url: https://example.com/group',
+        '    subcategories:',
+        '      - name: 前端开发',
+        '        groups:',
+        '          - name: 框架库',
+        '            subgroups:',
+        '              - name: 深层分组',
+        '                sites:',
+        '                  - name: DeepDefaultExternal',
+        '                    url: https://example.com/deep-default',
+        '                  - name: DeepExternalFalse',
+        '                    url: https://example.com/deep-false',
+        '                    external: false',
+        ''
+      ].join('\n'),
+      'utf8'
+    );
+
+    const config = loadConfig();
+
+    const groupSite = config.bookmarks.categories[0].groups[0].sites[0];
+    assert.equal(groupSite.external, true);
+
+    const deepSites = config.bookmarks.categories[0].subcategories[0].groups[0].subgroups[0].sites;
+    assert.equal(deepSites[0].external, true);
+    assert.equal(deepSites[1].external, false);
+  });
+});
+