feat: add power plants layer with WRI Global Power Plant Database

Map ~35,000 power generation facilities from 164 countries using the
WRI Global Power Plant Database (CC BY 4.0). Follows the existing
datacenter layer pattern with clustered icon symbols, amber color
scheme, and click popups showing fuel type, capacity, and operator.

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

frontend/src/app/page.tsx

@@ -163,6 +163,7 @@ export default function Dashboard() {
     internet_outages: false,
     datacenters: false,
     military_bases: false,
+    power_plants: false,
   });
 
   // NASA GIBS satellite imagery state