refactor: major architecture improvements

Security:
- Remove exposed NocoDB token from client code
- Add .env.example for environment variables

Shared Components:
- Carousel: touch/keyboard navigation, accessibility
- ChartCard: reusable chart container
- EmptyState: for no-data scenarios
- FilterControls: collapsible filter panel with reset button
- StatCard: metric display with change indicator
- ToggleSwitch: accessible radio-style toggle

Architecture:
- Create src/config/chartConfig.js for shared chart options
- Extract ChartJS registration to single location
- Reduce code duplication in Dashboard and Comparison

UX Improvements:
- Add empty state when filters return no data
- Add Reset Filters button to filter controls
- Add skeleton loader CSS utilities
- Improve focus states for accessibility
- Use shared components in Dashboard and Comparison

.env.example

@@ -0,0 +1,3 @@
+# NocoDB Configuration (optional - only needed if using NocoDB as data source)
+REACT_APP_NOCODB_URL=http://localhost:8090
+REACT_APP_NOCODB_TOKEN=your_token_here