Add master automation on/off switch to Settings

A single global toggle that overrides every zone's own auto_mode —
lets staff pause all HVAC automation stack-wide (e.g. during
maintenance) without touching each zone individually. Backed by the
existing config key/value store; pollOnce() short-circuits entirely
when off (no NewBook fetch, no state tracking, no device commands).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/types.ts

@@ -170,6 +170,7 @@ export interface NewbookSite {
 }
 
 export interface AppConfig {
+  automation_enabled?: boolean
   poll_interval_minutes: number
   default_arrival_time: string
   default_departure_time: string