feat: add VITE_MAINTENANCE_MODE env var for service screen

Set VITE_MAINTENANCE_MODE=true in .env to show an "Under Service"
screen and block all transactions. No hardware init, no Lightning
connection — just a static screen.

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

apps/machine/electron/main.ts

@@ -187,6 +187,9 @@ ipcMain.handle('get-config', () => {
 
     // Operator identity (comma-separated hex pubkeys)
     operatorPubkeys: process.env.VITE_OPERATOR_PUBKEYS || '',
+
+    // Maintenance mode — show "out of service" screen
+    maintenanceMode: process.env.VITE_MAINTENANCE_MODE === 'true',
   }
 })