feat: add hint to shrink window to hide config

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

src/components/ResponsiveTimeUntil.vue

@@ -141,6 +141,7 @@ const copyUrl = () => {
         <button @click="copyUrl">copy url</button>
         <a :href="url" target="_blank" rel="noopener noreferrer"><button type="button">open url</button></a>
       </div>
+      <p class="hint">Shrink the window to hide this config.</p>
     </form>
   </div>
 </template>
@@ -208,4 +209,12 @@ input {
     display: flex;
   }
 }
+
+.hint {
+  font-size: 0.75rem;
+  opacity: 0.5;
+  width: 100%;
+  text-align: center;
+  margin: 0;
+}
 </style>