diff --git a/public/app.js b/public/app.js
new file mode 100644
index 0000000..addc49f
--- /dev/null
+++ b/public/app.js
@@ -0,0 +1,192 @@
+/* app.js (v2.0.0)
+ * - loads /index.json (generated at build)
+ * - builds collapsible tree (left)
+ * - renders HTML in sandboxed iframe OR Markdown via marked+DOMPurify
+ * - default sort: date (new→old). toggle to alpha.
+ * - deep links: #=/posts/foo.md
+ */
+
+let INDEX = null;
+let CURRENT_PATH = null;
+let PATH_TO_EL = new Map();
+
+const treeEl = document.getElementById("tree");
+const sortSel = document.getElementById("sort");
+const filterSel = document.getElementById("filter");
+const iframe = document.getElementById("htmlFrame");
+const mdBox  = document.getElementById("mdContainer");
+const metaEl = document.getElementById("meta");
+
+const ICONS = {
+  ".md": "📝",
+  ".html": "🧩",
+  "dir": "🗂️"
+};
+
+function icon(ext){ return ICONS[ext] || "📄"; }
+
+function humanDate(ms){
+  const d = new Date(ms || 0);
+  if (!ms) return "";
+  return d.toISOString().slice(0,10);
+}
+
+function applySort(list, mode){
+  const arr = list.slice();
+  if (mode === "alpha") {
+    arr.sort((a,b)=> a.title.localeCompare(b.title));
+  } else {
+    arr.sort((a,b)=> (b.mtime||0)-(a.mtime||0)); // new → old
+  }
+  return arr;
+}
+
+function filterFlat(list, filter){
+  if (filter === "pinned") return list.filter(x=>x.pinned);
+  if (filter === "posts")  return list.filter(x=>!x.pinned);
+  return list;
+}
+
+function filterTree(node, allowedSet){
+  if (node.type === "file") return allowedSet.has(node.path) ? node : null;
+  const kids = [];
+  for (const c of node.children||[]){
+    const f = filterTree(c, allowedSet);
+    if (f) kids.push(f);
+  }
+  return { ...node, children: kids };
+}
+
+function buildNode(node){
+  if (node.type === "dir"){
+    const wrap = document.createElement("div");
+    wrap.className = "tree-node dir";
+    wrap.setAttribute("role","treeitem");
+    wrap.setAttribute("aria-expanded","false");
+
+    const label = document.createElement("div");
+    label.className = "dir-label";
+    label.innerHTML = `<span>${icon("dir")}</span><strong>${node.name}</strong>`;
+    label.addEventListener("click", ()=>{
+      const open = wrap.classList.toggle("open");
+      wrap.setAttribute("aria-expanded", open?"true":"false");
+    });
+    wrap.appendChild(label);
+
+    const children = document.createElement("div");
+    children.className = "children";
+    for (const c of node.children || []){
+      children.appendChild(buildNode(c));
+    }
+    wrap.appendChild(children);
+    return wrap;
+  } else {
+    const a = document.createElement("a");
+    a.className = "file";
+    a.setAttribute("role","treeitem");
+    a.href = `#=${node.path}`;
+    a.innerHTML = `${node.pinned ? '<span class="pin">PIN</span>' : ''}${icon(node.ext)} ${node.title} <span class="meta">${humanDate(node.mtime)} · ${node.name}</span>`;
+    a.addEventListener("click", (e)=>{
+      e.preventDefault();
+      openPath(node.path);
+    });
+    PATH_TO_EL.set(node.path, a);
+    return a;
+  }
+}
+
+function setActive(path){
+  document.querySelectorAll(".file.active").forEach(el=>el.classList.remove("active"));
+  const el = PATH_TO_EL.get(path);
+  if (el){
+    el.classList.add("active");
+    // ensure ancestors open
+    let p = el.parentElement;
+    while (p && p !== treeEl){
+      if (p.classList.contains("dir")){
+        p.classList.add("open");
+        p.setAttribute("aria-expanded","true");
+      }
+      p = p.parentElement;
+    }
+    el.scrollIntoView({ block:"nearest", behavior:"smooth" });
+  }
+}
+
+function rebuildTree(){
+  PATH_TO_EL.clear();
+  treeEl.innerHTML = "";
+  // compute filtered/sorted flat set
+  const sorted = applySort(filterFlat(INDEX.flat, filterSel.value), sortSel.value);
+  const allowed = new Set(sorted.map(x=>x.path));
+  const filteredTree = filterTree(INDEX.tree, allowed);
+  treeEl.appendChild(buildNode(filteredTree));
+  // expand top-level
+  treeEl.querySelectorAll(".dir").forEach(d=>d.classList.add("open"));
+}
+
+async function renderMarkdown(path){
+  iframe.hidden = true;
+  mdBox.hidden = false;
+
+  const res = await fetch(path, { cache: "no-cache" });
+  if (!res.ok) throw new Error(`fetch ${path} ${res.status}`);
+  const txt = await res.text();
+  // marked is global when CDN loads; fallback to plain text if missing
+  const rawHtml = (window.marked ? window.marked.parse(txt) : `<pre>${txt.replace(/[&<>]/g, s=>({ "&":"&amp;","<":"&lt;",">":"&gt;" }[s]))}</pre>`);
+  const safe = (window.DOMPurify ? window.DOMPurify.sanitize(rawHtml) : rawHtml);
+  mdBox.innerHTML = safe;
+}
+
+async function renderHTML(path){
+  mdBox.hidden = true;
+  iframe.hidden = false;
+  iframe.src = path;
+}
+
+async function openPath(path){
+  if (!path || path === CURRENT_PATH) return;
+  CURRENT_PATH = path;
+  if (location.hash !== `#=${path}`) history.pushState(null,"",`#=${path}`);
+
+  const entry = INDEX.flat.find(x=>x.path === path);
+  if (!entry) return;
+
+  metaEl.textContent = `${entry.pinned ? "Pinned • " : ""}${humanDate(entry.mtime)} • ${entry.path}`;
+  setActive(path);
+
+  if (entry.ext === ".md") await renderMarkdown(path);
+  else await renderHTML(path);
+}
+
+async function boot(){
+  try{
+    const res = await fetch("/index.json", { cache: "no-cache" });
+    INDEX = await res.json();
+  } catch (e){
+    treeEl.innerHTML = "<p style='color:#f66'>index.json missing. run the build.</p>";
+    return;
+  }
+
+  sortSel.addEventListener("change", rebuildTree);
+  filterSel.addEventListener("change", rebuildTree);
+
+  rebuildTree();
+
+  // choose initial route
+  const hashPath = location.hash.startsWith("#=") ? location.hash.slice(2) : null;
+  if (hashPath) {
+    openPath(hashPath);
+  } else {
+    // default: newest among current filter
+    const sorted = applySort(filterFlat(INDEX.flat, filterSel.value), "date");
+    if (sorted.length) openPath(sorted[0].path);
+  }
+
+  window.addEventListener("popstate", ()=>{
+    const hp = location.hash.startsWith("#=") ? location.hash.slice(2) : null;
+    if (hp && hp !== CURRENT_PATH) openPath(hp);
+  });
+}
+
+boot();
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