feat: add open url button to open countdown link in new tab

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/ResponsiveTimeUntil.vue

@@ -139,6 +139,7 @@ const copyUrl = () => {
       </div>
       <div>
         <button @click="copyUrl">copy url</button>
+        <a :href="url" target="_blank" rel="noopener noreferrer"><button type="button">open url</button></a>
       </div>
     </form>
   </div>