fix: HTML 预览子页面改用 JS 切换，避免沙箱 blob 导航被拦截

上一版通过设置 location.hash 触发 CSS :target 切页，但沙箱 blob
iframe 禁止导航到自身 blob URL，浏览器报 Not allowed to load local
resource，且 :target 仅在真实片段导航时更新，注定失效。改为完全
不动 hash，用 JS 模拟 html-multipage 技能的 .page/.pages 显示隐藏
（内联 display 覆盖类级 CSS），普通目录锚点仍平滑滚动。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend-web/src/core/utils/markdown.ts

@@ -102,14 +102,18 @@ function candidateTextFromHref(href: string): string {
  * Injects a small runtime script into generated HTML so that clicking an
  * in-page anchor (`<a href="#section">`) works inside the preview iframe.
  *
- * The hash MUST really change on click: multi-page documents (the
- * html-multipage skill) show/hide their sub-page `<section class="page">`
- * blocks with CSS `:target` rules, which only fire on a fragment navigation.
- * Intercepting with preventDefault + scrollIntoView alone leaves the hash
- * untouched, so sub-pages stay `display:none` and clicks look dead. The
- * handler therefore syncs `location.hash` (a same-document navigation —
- * safe in the sandboxed blob: frame) and only smooth-scrolls afterwards when
- * the target is actually rendered.
+ * The preview loads from a sandboxed `blob:` URL (opaque origin). Such a frame
+ * is FORBIDDEN from navigating to its own `blob:` URL, so changing the hash
+ * throws "Not allowed to load local resource: blob:…#page-x". And because CSS
+ * `:target` only updates on a real fragment navigation, the html-multipage
+ * skill's `:target`-based sub-page switching can never fire here.
+ *
+ * So we never touch the hash. Instead the handler emulates the skill's
+ * show/hide in JS: a "page" is a `.page` inside a `.pages` container; clicking
+ * a link to one shows it and hides its siblings (mirroring the
+ * `.page:target` + `~ #page-home` rules via inline `display`, which outranks
+ * the skill's class-level CSS). Plain in-page anchors (TOC headings, etc.) just
+ * smooth-scroll as before.
  */
 export function injectInPageAnchorHandler(html: string): string {
   const script = `<script>(function(){
@@ -119,6 +123,22 @@ export function injectInPageAnchorHandler(html: string): string {
     var esc=window.CSS&&CSS.escape?CSS.escape(id):id.replace(/["\\\\]/g,"\\\\$&");
     try{return document.querySelector("#"+esc+",[name=\\""+esc+"\\"]");}catch(e){return null;}
   }
+  function find(raw){
+    var id=decode(raw);
+    return document.getElementById(id)||document.getElementById(raw)||
+      document.getElementsByName(id)[0]||document.getElementsByName(raw)[0]||
+      bySelector(id)||bySelector(raw);
+  }
+  function switchPage(target){
+    var container=target.parentElement;
+    while(container&&!(container.classList&&container.classList.contains("pages"))){container=container.parentElement;}
+    var pages=(container||document).querySelectorAll(".page");
+    if(!pages.length)return false;
+    for(var i=0;i<pages.length;i++){pages[i].style.display="none";}
+    target.style.display="block";
+    if(window.scrollTo){window.scrollTo(0,0);}
+    return true;
+  }
   document.addEventListener("click",function(e){
     var el=e.target;
     while(el&&el.tagName!=="A"){el=el.parentElement;}
@@ -126,20 +146,10 @@ export function injectInPageAnchorHandler(html: string): string {
     var href=el.getAttribute("href");
     if(!href||href.charAt(0)!=="#"||href.length<2)return;
     e.preventDefault();
-    var raw=href.slice(1);
-    var id=decode(raw);
-    var target=
-      document.getElementById(id)||
-      document.getElementById(raw)||
-      document.getElementsByName(id)[0]||
-      document.getElementsByName(raw)[0]||
-      bySelector(id)||
-      bySelector(raw);
-    try{if(location.hash!=="#"+raw){location.hash=raw;}}catch(err){}
+    var target=find(href.slice(1));
     if(!target)return;
-    requestAnimationFrame(function(){
-      if(target.getClientRects().length){target.scrollIntoView({behavior:"smooth",block:"start"});}
-    });
+    if(target.classList&&target.classList.contains("page")&&switchPage(target))return;
+    target.scrollIntoView({behavior:"smooth",block:"start"});
   },true);
 })();</script>`;
   if (/<\/body>/i.test(html)) {