Redesign indicators panel: dynamic catalog, multi-instance, chart legend

- Build indicator list dynamically from class getMetadata() instead of hardcoded array
- Remove checkboxes; single-click previews config, double-click adds to chart
- Support multiple instances of same indicator type (unique IDs)
- Add TradingView-style column legend overlay on chart (top-left)
- Recalculate indicators when historical data is prefetched on scroll
- Make indicator list and config panels scrollable (hidden scrollbars)
- Remove AVAILABLE_INDICATORS from strategies/config.js

src/api/dashboard/static/js/indicators/atr.js

@@ -29,6 +29,7 @@ export class ATRIndicator extends BaseIndicator {
     getMetadata() {
         return {
             name: 'ATR',
+            description: 'Average True Range - measures market volatility',
             inputs: [{ name: 'period', label: 'Period', type: 'number', default: 14, min: 1, max: 100 }],
             plots: [{ id: 'value', color: '#795548', title: 'ATR' }]
         };