fix: dashboard — coverage pillar, active alerts env fallback

- Add coverage pillar to /api/health response and _serialize_health_score
- Add coverage to MeshHealth.pillars TypeScript interface
- Add Coverage PillarBar between Utilization and Behavior
- Active Alerts panel: show high-severity env events (immediate/priority)
  as fallback when mesh alerts are empty, with ENV badge

Issues 3 (Live Event Feed) and 4 (RF Propagation): diagnosed as
env feed configuration — SWPC adapter disabled, only ducting feed
loaded, /api/env/active returns 0 events. Not a code bug.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashboard-frontend/src/lib/api.ts

@@ -19,6 +19,7 @@ export interface MeshHealth {
   pillars: {
     infrastructure: number
     utilization: number
+    coverage: number
     behavior: number
     power: number
   }