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/icons-mode-from-site-yml.node-test.js

@@ -0,0 +1,49 @@
+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-icons-mode-test-'));
+  const originalCwd = process.cwd();
+
+  try {
+    process.chdir(tmpDir);
+    callback(tmpDir);
+  } finally {
+    process.chdir(originalCwd);
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  }
+}
+
+test('site.yml 的 icons.mode 应提升为顶层 icons.mode（manual 不应被默认 favicon 覆盖）', () => {
+  withTempCwd((tmpDir) => {
+    const defaultConfigDir = path.join(tmpDir, 'config', '_default');
+    fs.mkdirSync(defaultConfigDir, { recursive: true });
+
+    fs.writeFileSync(
+      path.join(defaultConfigDir, 'site.yml'),
+      ['title: Test', 'icons:', '  mode: manual', ''].join('\n'),
+      'utf8'
+    );
+
+    const config = loadConfig();
+    assert.equal(config.icons.mode, 'manual');
+  });
+});
+
+test('未配置 icons.mode 时应回退为默认 favicon', () => {
+  withTempCwd((tmpDir) => {
+    const defaultConfigDir = path.join(tmpDir, 'config', '_default');
+    fs.mkdirSync(defaultConfigDir, { recursive: true });
+
+    fs.writeFileSync(path.join(defaultConfigDir, 'site.yml'), 'title: Test\n', 'utf8');
+
+    const config = loadConfig();
+    assert.equal(config.icons.mode, 'favicon');
+  });
+});
+