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/bb.js

@@ -27,6 +27,7 @@ export class BollingerBandsIndicator extends BaseIndicator {
     getMetadata() {
         return {
             name: 'Bollinger Bands',
+            description: 'Volatility bands around a moving average',
             inputs: [
                 { name: 'period', label: 'Period', type: 'number', default: 20, min: 1, max: 100 },
                 { name: 'stdDev', label: 'Std Dev', type: 'number', default: 2, min: 0.5, max: 5, step: 0.5 }