feat: add debug information panel for data loading transparency

Added comprehensive debug panel that displays:
- Field mappings (CSV columns → item properties)
- Template placeholders (required template fields)
- CSV data preview (headers, sample data, validation warnings)

The panel appears below the timeline viewer and helps troubleshoot:
- Missing or mismatched CSV columns
- Template structure issues
- Data format problems

Shows warnings when no valid items are found with actionable
troubleshooting steps.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

index.html

@@ -325,6 +325,29 @@
     </div>
   </div>
 
+  <!-- Debug Information Panel -->
+  <div id="debugInfo" style="margin-top:16px; padding:16px; background:#f8f9fa; border:1px solid #e9ecef; border-radius:8px; display:none;">
+    <h3 style="margin:0 0 12px 0; font-size:14px; font-weight:600; color:#495057;">🔍 Debug Information</h3>
+
+    <!-- Field Mappings -->
+    <div id="fieldMappingInfo" style="display:none; margin-bottom:12px;">
+      <h4 style="margin:0 0 8px 0; font-size:13px; color:#6c757d; font-weight:600;">📋 Configured Field Mappings</h4>
+      <pre id="fieldMappingDisplay" style="background:#fff; padding:12px; border-radius:4px; border:1px solid #dee2e6; font-size:12px; margin:0; overflow-x:auto; font-family:'Courier New', monospace;"></pre>
+    </div>
+
+    <!-- Template Fields -->
+    <div id="templateFieldsInfo" style="display:none; margin-bottom:12px;">
+      <h4 style="margin:0 0 8px 0; font-size:13px; color:#6c757d; font-weight:600;">🖼️ Template Placeholders</h4>
+      <pre id="templateFieldsDisplay" style="background:#fff; padding:12px; border-radius:4px; border:1px solid #dee2e6; font-size:12px; margin:0; overflow-x:auto; font-family:'Courier New', monospace;"></pre>
+    </div>
+
+    <!-- CSV Data Preview -->
+    <div id="csvDataInfo" style="display:none;">
+      <h4 style="margin:0 0 8px 0; font-size:13px; color:#6c757d; font-weight:600;">📊 CSV Data Preview</h4>
+      <pre id="csvDataDisplay" style="background:#fff; padding:12px; border-radius:4px; border:1px solid #dee2e6; font-size:12px; margin:0; overflow-x:auto; font-family:'Courier New', monospace;"></pre>
+    </div>
+  </div>
+
   <script>
     document.addEventListener("DOMContentLoaded", () => {
       // Setup event handlers first