Redesign header warning notice with better styling

frontend/index.html

@@ -374,9 +374,10 @@
         </div>
 
         <!-- Web Notice -->
-        <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 style="text-align: center; line-height: 1.7; background: #fff8f0; border: 1.5px solid #f0a040; border-radius: 8px; padding: 8px 16px;">
+            <div style="font-size: 13px; font-weight: 700; color: #c05000;">⚠️ All data and analysis will be auto-deleted after 15 mins of inactivity.</div>
+            <div style="font-size: 12px; color: #555;">Please download your analysis when finished.</div>
+            <div style="font-size: 12px; color: #555;"><a href="https://github.com/caesium135/RodSizer.git" target="_blank" style="color: #c05000; font-weight: 600; text-decoration: underline;">RodSizer_local</a> is recommended for permanent preservation of data and analysis.</div>
         </div>
 
         <!-- About Button -->