Add demo mode: virtual hardware sandbox for testing without real devices

Demo mode provides a complete sandbox environment with:
- Virtual capture engine (radial rainbow test pattern on 3 displays)
- Virtual audio engine (synthetic music-like audio on 2 devices)
- Virtual LED device provider (strip/60, matrix/256, ring/24 LEDs)
- Isolated data directory (data/demo/) with auto-seeded sample entities
- Dedicated config (config/demo_config.yaml) with pre-configured API key
- Frontend indicator (DEMO badge + dismissible banner)
- Engine filtering (only demo engines visible in demo mode)
- Separate entry point: python -m wled_controller.demo (port 8081)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/static/js/features/tabs.ts

@@ -48,6 +48,8 @@ export function switchTab(name: string, { updateHash = true, skipLoad = false }:
             ? (localStorage.getItem('activeTargetSubTab') || 'led')
             : name === 'streams'
             ? (localStorage.getItem('activeStreamTab') || 'raw')
+            : name === 'automations'
+            ? (localStorage.getItem('activeAutomationTab') || 'automations')
             : null;
         _setHash(name, subTab);
     }
@@ -87,6 +89,7 @@ export function initTabs(): void {
         if (hashRoute.subTab) {
             if (saved === 'targets') localStorage.setItem('activeTargetSubTab', hashRoute.subTab);
             if (saved === 'streams') localStorage.setItem('activeStreamTab', hashRoute.subTab);
+            if (saved === 'automations') localStorage.setItem('activeAutomationTab', hashRoute.subTab);
         }
     } else {
         saved = localStorage.getItem('activeTab');