Summary

Analysing an ANR trace revealed that MonitorNotifications held a single
shared mutable NotificationCompat.Builder object, protected by a
coroutine Mutex. This had two problems:

• getStartupNotification() bypassed the lock entirely, silently violating
  the synchronization protocol
• After a device-connected call, switching to a null-device call could leave
  stale CustomBigContentView content on the shared builder

Changes

• Remove shared builder field and builderLock (Mutex) entirely
• Add private val openPi: PendingIntent (promoted from init local)
• Add private fun baseBuilder(channelId: String) helper for common setup
• getBuilder() now takes channelId and returns a fresh builder each call
• getNotification() and getNotificationConnected() are no longer suspend
  (Mutex removed, no coroutine context needed)
• All three public methods produce functionally identical notifications

Test plan

• Build passes: assembleFossDebug, assembleGplayDebug
• Startup notification shows "no pods" title and taps open MainActivity
• Device notification updates correctly as pods connect/disconnect