Fix issues when the system kills the app during playback (#1116)

## Description
This fixes some issues restoring the app state after it is killed by the
system, such as to free up memory while the screensaver is showing.

### Dev notes

Reproduce:
1. Start playing media from beginning
2. Pause at 10 minute mark
3. Wait for screensaver to start
4. Press a button
5. Media starts from beginning

Note: It may take a few iterations of 2-4 before the OS destroys the
app.

If the app is killed during playback, ideally Wholphin restores state to
_previous_ page, not playback since restoring playback is complex.
Before this PR, the back stack was maintained by Compose's
`rememberSerializable` and would be restored _after_ the
`PlaybackLifecycleObserver` cleans up the playback destination.

This meant that the app would be restored, from scratch, to the playback
page. The `PlaybackViewModel` would be initialized from scratch as it
was originally called, ie play from beginning.

The fix is to save and restore the back stack outside of Compose.

### Related issues
Fixes #767

### Testing
Emulator & nvidia shield

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/MainContent.kt

@@ -33,10 +33,8 @@ import com.github.damontecres.wholphin.services.ScreensaverService
 import com.github.damontecres.wholphin.services.SetupDestination
 import com.github.damontecres.wholphin.ui.components.AppScreensaver
 import com.github.damontecres.wholphin.ui.nav.ApplicationContent
-import com.github.damontecres.wholphin.ui.nav.Destination
 import com.github.damontecres.wholphin.ui.setup.SwitchServerContent
 import com.github.damontecres.wholphin.ui.setup.SwitchUserContent
-import com.github.damontecres.wholphin.ui.util.ProvideLocalClock
 
 @Composable
 fun MainContent(
@@ -45,7 +43,6 @@ fun MainContent(
     appPreferences: AppPreferences,
     backdropService: BackdropService,
     screensaverService: ScreensaverService,
-    requestedDestination: Destination,
     modifier: Modifier = Modifier,
 ) {
     Surface(
@@ -113,7 +110,6 @@ fun MainContent(
                                 ApplicationContent(
                                     user = current.user,
                                     server = current.server,
-                                    startDestination = requestedDestination,
                                     navigationManager = navigationManager,
                                     preferences = preferences,
                                     modifier = Modifier.fillMaxSize(),