feat(biochem): skeleton loaders for async fetches

Replace plain "Загрузка..." placeholders with shimmer-animated skeletons
matching the actual layout shape:
- library: 12 placeholder cards (canvas + 2 lines)
- reactions: 6 row skeletons (stripe + title + 2 text lines)
- properties: 10 sidebar row shimmers (thumb + 2 lines)
- biochem editor: 4-5 row skeletons for saved-molecules and challenges lists

No existing skeleton classes in ls.css; added local .bc-sk-* helpers per page.

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

frontend/biochem-properties.html

@@ -155,6 +155,27 @@
     .add-card:hover { border-color: rgba(155,93,229,.5); color: #9B5DE5; background: rgba(155,93,229,.05); }
     .add-card-icon { font-size: 2rem; }
 
+    /* ── Shimmer skeleton ── */
+    @keyframes bc-shimmer {
+      0%   { background-position: -200% 0; }
+      100% { background-position:  200% 0; }
+    }
+    .bc-sk {
+      background: linear-gradient(90deg,
+        rgba(255,255,255,0.04) 0%,
+        rgba(255,255,255,0.10) 50%,
+        rgba(255,255,255,0.04) 100%);
+      background-size: 200% 100%;
+      animation: bc-shimmer 1.6s infinite;
+      border-radius: 8px;
+    }
+    .bc-sk-line { height: 10px; margin: 5px 0; }
+    .bc-sk-line.sm { width: 55%; }
+    .bc-sk-line.md { width: 80%; }
+    .bc-sk-molrow { display: flex; align-items: center; gap: 8px; padding: 7px 8px; margin-bottom: 2px; }
+    .bc-sk-molrow .bc-sk-thumb { width: 34px; height: 34px; border-radius: 8px; flex-shrink: 0; }
+    .bc-sk-molrow .bc-sk-info { flex: 1; }
+
     /* ── Mobile ── */
     @media (max-width: 768px) {
       html, body { overflow: auto; }
@@ -193,7 +214,7 @@
             <button class="ps-cat" data-cat="amino_acid" onclick="setCat(this,'amino_acid')">АК</button>
           </div>
         </div>
-        <div class="ps-list" id="mol-list"><div style="padding:20px;color:#555;font-size:.8rem">Загрузка…</div></div>
+        <div class="ps-list" id="mol-list"></div>
       </div>
 
       <!-- Right: comparison -->
@@ -294,7 +315,19 @@ let _catFilter = '';
 let _searchQ = '';
 let _compare = []; // array of mol objects, max 4
 
+function bcSkMolList(n = 10) {
+  return Array.from({length: n}, () => `
+    <div class="bc-sk-molrow">
+      <div class="bc-sk bc-sk-thumb"></div>
+      <div class="bc-sk-info">
+        <div class="bc-sk bc-sk-line md"></div>
+        <div class="bc-sk bc-sk-line sm"></div>
+      </div>
+    </div>`).join('');
+}
+
 async function init() {
+  document.getElementById('mol-list').innerHTML = bcSkMolList(10);
   try {
     _allMols = await LS.biochemGetMolecules();
     applyFilter();