fix conversion layout (button below input), add missing closing divs, add HTML div-balance test

tests/html.test.js

@@ -0,0 +1,12 @@
+const { test } = require('node:test');
+const assert = require('node:assert');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const html = fs.readFileSync(path.join(__dirname, '..', 'index.html'), 'utf8');
+const opens = (html.match(/<div\b/g) || []).length;
+const closes = (html.match(/<\/div>/g) || []).length;
+
+test('HTML <div> tags are balanced', () => {
+    assert.strictEqual(opens, closes, `Unbalanced divs: ${opens} opens vs ${closes} closes`);
+});