Improve header notice visibility and accuracy

frontend/index.html

@@ -374,9 +374,9 @@
         </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 style="text-align: center; font-size: 13px; color: #b05000; line-height: 1.6; font-weight: 500;">
+            ⚠️ Web demo only — all uploads and results are <strong>temporary</strong>.<br>
+            Data is lost when the server restarts. For research use, please download the <a href="https://github.com/caesium135/RodSizer" target="_blank" style="color: #b05000;">local version</a>.
         </div>
 
         <!-- About Button -->