Add file upload feature: Load JSON data from disk

- Add 'Load from Disk' button to toolbar in middle section
- Implement file selection dialog with JSON validation
- Support .json and .txt file formats
- Display error messages for invalid JSON files
- Update README to document the new file upload capability
- Add styling for the new upload button
- Feature matches requirement in copilot-instructions.md

src/index.css

@@ -59,6 +59,12 @@ code {
   color: #155724;
 }
 
+.btn-outline-success:hover {
+  background-color: #28a745;
+  border-color: #28a745;
+  color: white;
+}
+
 .header-section {
   background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
   color: white;