revert iframe approach, restore vanilla JS peek + re-add resize handles

Reverted the iframe-based peek panels (d13f137, 99fee56) which caused:
- Sidebar/header still visible inside iframe despite embed CSS
- Resize handle broken by iframe layout

Restored:
- Vanilla JS content loading via API (_loadPreviewContentVanilla)
- Peek-body div instead of iframe
- Resize handles on both library and local-workspace panels
  (click=close, drag=resize, width persisted in localStorage)

app/templates/library.html

@@ -407,6 +407,7 @@
 
 <!-- ── Side Peek Panel ── -->
 <div class="peek-overlay" :class="{ open: peekItem !== null }" @click.outside="closePeek()">
+  <div id="lib-peek-resize-handle" style="position:absolute;left:-4px;top:0;width:8px;height:100%;cursor:col-resize;z-index:501;" title="Drag to resize — click to close"></div>
   <template x-if="peekItem">
     <div style="display:flex;flex-direction:column;height:100%;">
       <div class="peek-header">
@@ -1303,5 +1304,40 @@ function libraryPage() {
     }
   };
 }
+// Init peek resize handle
+(function() {
+  var panel = document.querySelector('.peek-overlay');
+  var handle = document.getElementById('lib-peek-resize-handle');
+  if (!panel || !handle) return;
+  var startX, startWidth, dragging = false;
+  handle.addEventListener('mousedown', function(e) {
+    e.preventDefault();
+    startX = e.clientX;
+    startWidth = panel.offsetWidth;
+    dragging = true;
+    document.body.style.userSelect = 'none';
+  });
+  document.addEventListener('mousemove', function(e) {
+    if (!dragging) return;
+    var deltaX = startX - e.clientX;
+    var newWidth = Math.max(300, Math.min(window.innerWidth * 0.9, startWidth + deltaX));
+    panel.style.width = newWidth + 'px';
+  });
+  document.addEventListener('mouseup', function(e) {
+    if (!dragging) return;
+    document.body.style.userSelect = '';
+    var moved = Math.abs(e.clientX - startX);
+    if (moved < 3) {
+      var el = document.querySelector('.peek-overlay');
+      if (el && el.__x) { el.__x.$data.peekItem = null; }
+    }
+    dragging = false;
+    try { localStorage.setItem('fd_peek_width', panel.style.width); } catch(ex) {}
+  });
+  try {
+    var saved = localStorage.getItem('fd_peek_width');
+    if (saved) panel.style.width = saved;
+  } catch(ex) {}
+})();
 </script>
 {% endblock %}