Add web demo notice with session/storage warning in header

frontend/index.html

@@ -373,6 +373,12 @@
             </div>
         </div>
 
+        <!-- Web Notice -->
+        <div style="text-align: center; font-size: 11px; color: #888; line-height: 1.5;">
+            ⚠️ Web demo — uploads are session-based and auto-deleted after 24h.<br>
+            For persistent use, download the <a href="https://github.com/caesium135/RodSizer" target="_blank" style="color: #888;">local version</a>.
+        </div>
+
         <!-- About Button -->
         <button class="about-btn" onclick="openModal()" title="About & Credits">ℹ</button>
     </div>