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

index.html

@@ -535,7 +535,7 @@
 				<option value="plmn">MCC,MNC → PLMNsel</option>
 				<option value="plmnwact">MCC,MNC,AcT → PLMNwAcT</option>
 			</select>
-			<div class="flex gap-2 mb-2">
+			<div class="flex flex-col gap-2 mb-2">
 				<input id="conv-input" class="font-mono flex-1 border border-gray-300 dark:border-slate-600 text-sm rounded px-2 py-1.5 dark:bg-slate-800">
 				<button onclick="genConvert()" class="px-3 py-1.5 bg-blue-600 text-white text-xs font-medium rounded hover:bg-blue-700" data-l10n="Convert">Convert</button>
 			</div>
@@ -564,6 +564,7 @@
 			<textarea id="conv-result" rows="3" readonly class="w-full font-mono border border-gray-300 dark:border-slate-600 text-xs rounded px-2 py-1.5 mt-2 bg-white dark:bg-slate-800"></textarea>
 		</div>
 	</div>
+</div>
 </div>
 
 	<div id="tab-sp" class="tab-content hidden">