fix(audio): release COM wrappers and adopt singleton view-model lifetime

[hoobio]

Summary

Fixes a runaway thread/memory leak that grew to ~32k threads and ~4 GB private bytes over an 11-hour session. Three coupled causes, all addressed:

• AudioSessionService.BuildSnapshot leaked an MMDevice and an AudioSessionControl per audio session per call. Both now disposed in finally blocks.
• SessionWatcher.OnSessionCreated registered an IAudioSessionEvents sink on each new session and never unregistered it. Sinks are now tracked in a ConcurrentBag and unregistered + disposed on watcher disposal, with a _disposed guard so a late COM callback no-ops instead of registering against a dying watcher.
• AudioEndpointService.BuildList had the same MMDevice non-disposal pattern. Same finally-block fix.
• HostBuilderExtensions registered the three view models and their pages as Transient. Direct Frame.Content navigation never disposed them, so each page switch leaked a VM that stayed subscribed to audio events forever. Switched to Singleton: one instance per type for the app lifetime, disposed via the host on shutdown. As a side benefit, page state is preserved across navigation.

Test plan

• Builds clean (0 warnings, 0 errors).
• Manual: launched fresh, baseline ~108 threads / ~190 MB.
• Manual: rapid Sessions <-> Rules switching no longer grows memory (was +160 MB in 4 minutes pre-fix; flat after fix).
• Overnight soak: leave running 8+ hours and confirm thread count stays under ~200 and working set under ~300 MB. (Pre-fix this was the path to 32k threads.)

fix(audio): release MMDevice / AudioSessionControl wrappers and unregister event sinks
fix(app): use singleton lifetime for pages and view models so navigation does not leak VMs