feat: add skeleton loading state for alt-IP section when primary IP is IPv6

Show a shimmer skeleton placeholder while fetching the local IPv4
address via the IPv4-only domain. When the primary IP is already IPv4,
no loading state is displayed and the alt section fades in directly.

static/script.js

@@ -272,10 +272,37 @@ function initCopyIp(wrap){
     return html;
   }
 
-  function showAltIp(ip,info){
-    section.innerHTML=buildBlock(ip,info);
+  function buildSkeleton(){
+    var html='<div class="skeleton-block">';
+    html+='<div class="skeleton-bone skeleton-ip"></div>';
+    html+='</div>';
+    html+='<div class="skeleton-table">';
+    for(var i=0;i<8;i++){
+      html+='<div class="skeleton-row">';
+      html+='<div class="skeleton-row-label"><div class="skeleton-bone"></div></div>';
+      html+='<div class="skeleton-row-value"><div class="skeleton-bone"></div></div>';
+      html+='</div>';
+    }
+    html+='</div>';
+    return html;
+  }
+
+  function showSkeleton(){
+    section.innerHTML=buildSkeleton();
     section.hidden=false;
-    initCopyIp(section.querySelector(".ip-copy-wrap"));
+    section.classList.add("alt-visible");
+  }
+
+  function showAltIp(ip,info){
+    section.classList.remove("alt-visible");
+    setTimeout(function(){
+      section.innerHTML=buildBlock(ip,info);
+      initCopyIp(section.querySelector(".ip-copy-wrap"));
+      section.hidden=false;
+      // Force reflow before adding the visible class for transition
+      void section.offsetHeight;
+      section.classList.add("alt-visible");
+    },primaryIsIPv6?300:0);
   }
 
   function fetchJson(ip){
@@ -285,8 +312,17 @@ function initCopyIp(wrap){
     });
   }
 
+  function hideSkeleton(){
+    section.classList.remove("alt-visible");
+    setTimeout(function(){
+      section.innerHTML="";
+      section.hidden=true;
+    },300);
+  }
+
   function detectAltIp(){
     if(primaryIsIPv6&&ipv4Domain){
+      showSkeleton();
       var controller=new AbortController();
       var timeout=setTimeout(function(){controller.abort();},5000);
       fetch("https://"+ipv4Domain+"/",{signal:controller.signal})
@@ -297,12 +333,12 @@ function initCopyIp(wrap){
         })
         .then(function(text){
           var ip=text.trim();
-          if(!ip||ip.indexOf(":")!==-1)return;
+          if(!ip||ip.indexOf(":")!==-1){hideSkeleton();return;}
           return fetchJson(ip).then(function(info){
             showAltIp(ip,info);
           });
         })
-        .catch(function(){});
+        .catch(function(){hideSkeleton();});
     }else if(!primaryIsIPv6){
       fetch("/ip")
         .then(function(r){

static/style.css

@@ -477,6 +477,82 @@ main {
   padding: 6px 10px 6px 28px;
 }
 
+#alt-ip-section {
+  opacity: 0;
+  transition: opacity 0.3s ease;
+}
+
+#alt-ip-section.alt-visible {
+  opacity: 1;
+}
+
+.skeleton-block {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+}
+
+.skeleton-bone {
+  background: var(--color-gray5);
+  border-radius: 6px;
+  position: relative;
+  overflow: hidden;
+}
+
+.skeleton-bone::after {
+  content: "";
+  position: absolute;
+  inset: 0;
+  background: linear-gradient(
+    90deg,
+    transparent 0%,
+    var(--color-bg-tertiary) 50%,
+    transparent 100%
+  );
+  animation: skeleton-shimmer 1.5s ease-in-out infinite;
+}
+
+@keyframes skeleton-shimmer {
+  0% { translate: -100% 0; }
+  100% { translate: 100% 0; }
+}
+
+.skeleton-ip {
+  inline-size: 260px;
+  block-size: 42px;
+  margin-block: 28px 20px;
+}
+
+.skeleton-table {
+  inline-size: 100%;
+}
+
+.skeleton-row {
+  display: flex;
+  align-items: center;
+  padding-block: 10px;
+  border-block-start: 1px solid var(--color-separator);
+}
+
+.skeleton-row:last-child {
+  border-block-end: 1px solid var(--color-separator);
+}
+
+.skeleton-row-label {
+  inline-size: 42%;
+  padding-inline: 35px 32px;
+}
+
+.skeleton-row-label .skeleton-bone {
+  inline-size: 60px;
+  block-size: 15px;
+}
+
+.skeleton-row-value .skeleton-bone {
+  inline-size: 100px;
+  block-size: 15px;
+}
+
 @media (width <= 480px) {
   main {
     margin-block-start: 40px;