feat: add global toggle to turn off background scanning and connection loops

[Falak-Parmar]

Description

This pull request implements a global AirSync Active toggle feature that allows users to completely turn off the background connection and discovery services, resolving the background scanning behavior discussed in #131.

Key Changes

1. Persistent Preference: Added an app_enabled preference key (defaulting to true) inside DataStoreManager.kt.
2. Dashboard UI Toggle:
   • Implemented a two-state dashboard in AirSyncMainScreen.kt.
   • OFF State: Displays a clean, minimal landing layout with the monochrome AirSync logo and a large "Turn On AirSync" button.
   • ON State: Displays the active dashboard, with a clean "AirSync Active" toggle row at the very top of the list for quick access.
3. Service Management & Safety:
   • Intercepted service starts in ServiceManager.shouldServiceRun() so that background scans/UDP/mDNS do not execute when toggled off.
   • Fixed potential Android OS crashes (ForegroundServiceDidNotStartInTimeException) in AirSyncService.onStartCommand() by ensuring startForeground() is called before stopSelf() in the early-exit guard.
   • Avoided calling startForegroundService() entirely in the service helper if the app is disabled.
4. Connection Blockers:
   • Added guards inside WebSocketUtil.connect() and WebSocketUtil.requestAutoReconnect() to reject any incoming/outgoing manual or automatic WebSocket connection requests when the setting is toggled off.

References

• Fixes [Feature Request] Add a global "AirSync Active" switch to completely stop background scanning and connection loops #131